I'll answer generally as specifics would take me forever.
1) The misfires are random - ie: sometimes on the same cylinders, but sometimes not
2) The only real constant is that I don't get the misfires until the car is driven close to 10 miles (a little less if it's hot out side) - it seems to be temperature dependent.
* Bad ground is a real possibility and I have so far checked the engine ground by the rear passenger side wheel. Cleaned both ends and both sides (engine and chassis) lugs and studs and re-tightened. No love...
3) Fuel? Mmmmm, I don't think so (although anything is possible) - I always use Shell V nitro plus 93 and mostly from the same station (I'm almost positive that at least the last 5 fills have been from the same station).

How's about have you checked,cleaned,retightened ground points #10 and 11?
If that doesn't work ring out ignition coil sockets between X59/1 connector and coils. If that checks then pin side of connector to DME plug. Looking for faulty or broken down wires.
Possibly a fault of some sort will show up.

^^^This is what I'm working on now...

 

^^^ Interestingly, after cleaning and tightening the passenger side engine ground point #11 (no change in misfires) I moved over to the drivers side to check #10... ... ... Hmmm, no ground point #10! There is clearly a lug on the engine block and a point on the frame near the bottom of the motor mount. The point on the frame was pretty rusted and the stud was missing - it looked like someone had put some kind of sealer/goop on it. I cleaned it out and tapped it the best I could with a M8 1.25 tap. It was super difficult to get the tap wrench up the the spot... I finally went to Lowe's and got a tap wrench that allows the tap to be driven by a socket or standard ratchet handle with a long extension on it. Unfortunately, they didn't have a flat bottom/flush cut 8mm 1.25 tap. I'll probably get one a NAPA tomorrow (Advance didn't have it either - only a 8mm 1.0 ). In any case, I'm not expecting this to cure my problem as I'm sure there's never been a ground at this point (I had the engine out a few years ago before the full rebuild). According to the Bentley manual there are at least two more ground points in the engine bay - I don't remember seeing these either when I dropped the engine... I'm also planning on going back and checking the grounds at the ECU relay are as well...

 

Same here. Removes the cats, problem solved.

 

Wow! Interesting... seems bizarre, but the fact that I haven’t been able to figure this out yet is also bizarre. What did you replace the cats with?

 

Also bizarre is that I opened up X59 plug 1 and found that I’m missing the connection on pin/socket 11 (valve lift control cyl 4-6) strange no?

 

Nothing!

 

^^^ Not the case - I was reading the wrong page (The EWD pages are many, and slightly confusing) - I was looking at Motronic 7.8 when I should have been looking at 7.2... In any case, I won't be able to get back to it until tomorrow. Will post back then. Not sure if I want to drive around without cat's - how loud is it? Please compare to stock with cats (as opposed to "it's not loud at all")...

 

I have these these 200 cell cats on my '99 C2 with FSI 3.6 Street Performer. I also have their headers. Along with the install I replaced the O2s with the Bosch Universals from Warehouse33. Not a bit of trouble with any of it so far with over 10k miles logged.
https://www.topgear-eurosport.com/po...ll-sports-cats
You would be in at about $750 for the CATS and the O2s.

 

I'd probably be ok with something like that^^ if I'm sure that's my problem - the difficulty is in figuring it out... I don't want to spend $800 to find out I've still got this misfire problem...

 

Oh, yes of course.
I'm just sharing this, should the CATS be your problem. It is not a terribly expensive route if you have to go down that road.

 

Maybe someone on the forum can loan you a pair of CATless pipes or hollowed out pair of stockers.

 

I can't speak to whether this is actually your problem, but I'll give a heads-up on the TopGear cats. (2004 C4S) My factory CATS were bad when I got the car (after some misfire issues that I fixed), and they rattled when I took them off, so definitely a problem. I then bought and installed these same TopGear Cats back in January of this year. They are beautifully made, nice welds, very light weight, and installed easily. I also installed 4 new Bosch O2 sensors at the same time.
Within 30 miles, and continually after that, I got Cat efficiency codes for both bank 1 and bank 2. Used Durametric to monitor voltages, and the sensors are good, the Cats just don't do enough to attenuate the oxygen level. I tried to contact TopGear asking what they generally recommended, but I could never get them to reply. I had to install O2 spacers onto my after-Cat sensors. Finally got to that a month ago, and the code is finally cleared and gone.

You can search the forums, some people haven't had an issue, but many have. If you DO have a Cat issue, my recommendation would be to either:
1) Be honest about it and just use a straight/test pipe and O2 spacers. The TopGear Cats really aren't doing much anyway.
2) Pay more for better Cats. (Would have been my choice, since I generally like having breathable air on the planet.)

Good luck!

 

So far I’ve : cleaned & checked all engine ground connections (gp10 & 11) cleaned & checked ECU connections, cleaned and checked coil grounds and toned out all coil green wires to X59-1 plug (black) in engine bay. Unfortunately, I’ve only been able to do this while the engine is cool And of course the engine runs great while it’s cool 😩... How about the injectors? I suppose I should check for power to them as well... once again, not sure how I can pull this off when the engine is hot??? I’d be grateful for any ideas... Thanks

 

I seem to remember Jake mention injectors acting up (leaking) when hot. Not sure if there's anyway to test them on your end though. Maybe try swapping some around and see if anything changes?