My JPS was scary .....till I added caster as advised by the guys in here and lowered the tire pressures to 20 psi....I thought about putting a couple sand bags in the trunk but didn't do that.... it is fine at 85 mph, any more than that is a "come to jesus moment".
I also had a 69 911 and it was very stable at top speed....it very well could have had the weights in the front as Wolfgang pointed out......but I wasn't aware of them.
Compared to any other cars I owned at that time, including Camaros and Mustangs and Corvettes, that Porsche was on rails....