Interesting one here and have been doing a bunch of research with incredible information however most peoples problems seem to be the reverse of mine; cold start problems/running where mine is problems once warm.
Takeaways and info.
Brief history - have owned the cars for 2 years and love it, no problems whatsoever. Always starts, runs great, etc... the one we wanted and truly enjoy it.
I have a branch office about 60 miles away and take it weekly for a run through the gears and warm up, never fails and enjoy the time spent behind the wheel.
On one of my trips two months ago I was cruising along and a hesitation started, following by a bit of rough running. 60-70MPH, 2000-2200rmp. It felt like it started missing, less power and if I could keep RPM's up it wouldn't be too noticeable however this came with lack of the power I was used to; again best way to put it is a miss. I pulled over and check plugs and sure enough the boot over the forward passenger spark plug was loose. I pushed it down, fired up and cruised home with no issues.

Fast forward two months and everything has been great.
Last week I drive again to the branch office and 10 mins into the drive there is the hesitation and loss of power again. Well I know what this is and check the boot but its secure... Starts back up fine and another 10 mins passes and symptoms come again. Pull over, turn off... wait 5 mins fires right up, no problems. Onward and this pattern occurred until I reached my office. I let it set for 3 hours and took it to dinner, 5 mins away and no problems. I stayed the night, ran an errand in the morning no problem.
I needed to get back so thinking the problem is resolved I start my trip home. Almost identical to the day before everything is fine for about 10 mins, cruising right along and...miss, another, hesitate when throttling, etc... Stopped 3 times, each for about 10 mins and was able to make it about the same time each with no problems until I got home.

A couple more interesting pieces of info now that problem is consistent:
Car always starts no problem whether cold or warm
When cold the car immediately idles great and runs like normal
When the issue is happening and I turn the car off, the car then does not hesitate to start however the rpm's lobe up and down, almost to the point of killing the car when they are low. I need to give it gas to keep it going.
*I went to a gas station I have never visited PRIOR to my trip up to the branch office (as in on my way out of town) and filled up*

Intial thought would be:
Drive existing tank of gas down
Fuel cleaner/octane add, refill with known normally used gas station fuel - determine if noticeable change of performance
Some basic research (again a lot of "cold start" problems) point to replacement/check of:
Vacuum lines
Injector Seals
Cylinder head temperature sender
Fuel pump relay

Not sure if anyone has seen these symptoms or has any additional info but I will update post fuel change and fuel treatment and let everyone know.

 

zingerroo,

I went though this with my SC last year -exactly this problem.

In my case the problem was a failing rebuilt CDI box. Prior to determining this I trouble shot whether it was fuel or spark. Confirmed there were no fuel issues, then changed plug wires, cap, rotor and distributor with no improvement. Next I ordered a New CDI box with harness and the car now runs perfect. The stock CDI boxes are Bosch but new ones are NLA as far as I know. You can buy rebuilt ones, but like my experience with a rebuilt one, sometimes they are not fully rebuilt. I put in a new box made by Partsklassic. Excellent reputation and service. A quality CDI. Get their harness as well. Good luck!

 

I agree, sounds like the CD box is failing

 

got any sc buddies ? its 5 minutes to swap