hi ,i would start with ..are the lug nut tight???
Ivan

Well... seeing as I don't have the equipment to remove the tire from the wheel, its not quite as easy as that. My tires are directional, so its not a simple as as "swap them back"....or else I would have ;)

Shows you how excellent my reading comprehension is.

So, if they're uni-directional and rotated on the wheels, what was the inside on the left front is now the outside on the right front, correct? There's a real good chance that is indeed exactly your problem, especially if nothing else was changed.

Can you do a tread depth measurement comparing maybe 1" inboard of the inner and outer shoulder? I guess if your tires are slightly cone shaped from the previous mounting it kind of would have the effect of changing kingpin incidence. At least it works that way theoretically in my head.

Good idea, ill compare thread depths when I back at the car. Your theory is what I suspected.

Thanks!

I had this problem after putting new Pirelli 6000s on the rear of my 1978 SC. Didn't settle down until I put Pirelli 6000s on the front as well. Was quite scary above 70 mph; car moved from side-to-side at will.

I respectfully disagree with the idea that the problem is moving the tires on the front of the car. Your thought of putting better rubber where it was once worn can only improve traction. I think tire pressure could be an issue, or it is a suspension part that is worn, and you just did not notice it before. So check the tire pressure, take a ride, then Jack the wheels up and look for bearing, tie rod, rack, ball joint, shock wear.

It comes up from time to time that tires on 911s are best replaced in sets of 4.

Rotate them back, check the front end, etc., and consider replacement. These tires are not incredibly expensive. You only have two feet, but you wouldn't replace just one sneaker.