Porsche knew they had a high-speed stabililty problem very early off. Based on what I've read (will search for article), it was one of the first post-production development projects (ca. 1978), which included further wind-tunnel testing.
Worth noting that near when the 'S' spoiler was introduced, so was adjustable front spring collars, partly to reduce front end lift by reducing how much air could pass under the car.

I suspect that knowing this problem would not affect many, Porsche did not issues a TSB (at least not one I could find).

 

How high is "high speed"? My '79 had an S4 style wing on it when I bought it. I loved the way it "sat down" above 90 mph. The wing is now gone, and I have few opportunities to take it above 90 for any length of time. I don't know if I've lost that.

 

The front steering starts to get very light over 115mph, based on my experience. This also included some headwind, but not exclusively.
Ran it up to 130mph sustained, but it was not confidence inspiring.

Note: there was a fair amount of travel weight in the back hatch, futher lifting the front. This may have exacerbated the effect.

 

This is the first time I see a car just like mine, including the green interior & pascha. Nice too see hw it's upposed to look like eventually!

 

That green car is amazing!

Ad0911, green cars look the best. It was after all Ferry's favorite color on all his Porsche's. You have great taste.

 

That green car has what looks like fed markers on the back but standard US markers on the front.

 

The green car is a Euro VIN 9288101034

 

Just like the Audi TT and VW Bug..... Funny how the Turbo Bug's spoiler doesn't deploy until something like 90mph

I don't think I've ever driven a spoiler-less 928 at speed. My 79 has an S front spoiler with brake ducts, no rear spoiler.

125+ on the track it feels fine. But it's also a few inches lower than stock.

 

The Project 928 book has numbers about the "lift" at speed it was like LESS than 120lbs as I recall so the other 3200 lbs of weight holds the car down. The old very brown 1980 on the highbanking at Fontana is rock solid Big Willow it works just fine with no stinking spoilers !

 

It must be your car Jim - I frequently drove Blumax at sustained speeds of 130+ (as high as 140 on the speedo) and never felt the slightest twitch of uncertainty. As JB says, rock solid. That was good Montana roads too. Can't do that around here. But I've had no problems at speed, even in wind, with no spoilers.