lol my 289 has trouble going over 80 so no problems here haha.

 

Don' ferget 'bout dah tars. All the above posts are dead on, but I didn't see one mention of the tires. Low end, old, bias, any or all of these combos, can also be a factor.

 

Worn parts, soft suspension, bad alignment, crappy tires.

Suspension is actually a big factor, soft suspension will allow the front of the car to more around more.

I've had mine up to 140mph and it's completely stable, but that's with the 1" a-arm drop, 620 springs, KYB Gas-A-Just shocks, 1.125" sway bar and all poly bushings, with good tires.

 

Wow 620 springs? I was just thinking of backing off of my 450 to 400s on my TCP coil over setup.

Lighter springs and easy to extend shocks will allow the nose of the car to come up at speed which lets more air under the car and can add to instability. You want some movement but lower is often better as long as you still have travel.

Yeah, when I was running bias ply tires, my car wondered at every little change in the road surface. That is because the tire is much more flexible than the new radials.

Definitely check out the bushings and tie rods. That is just a safety thing.

 

Mine had a bad lower control arm that would move back and forth at the bottom of the shock tower. You might check into that.

 

I'm having the same issue with my 2011 Premium Convertible (6 cyl). I switched out the tires to BFG T/A's and have a much smoother ride, but still front end lift at 80 mph without fail on any freeway driving surface. It reminds me of the drift I had on an '87 Z28. I haven't owned an American care since then. It seems like it must be a weight and suspension issue unique to these light American pony cars.

 

BFG Radial TA tires on a 2011 Mustang? I have not seen that before. Also, I wouldn't run those tires on any car. Its a lousy tire.

 

for future reference you might get a better answer posting in the 2010-2011 v6 section. ive only heard of the newer mustang being floaty at 120+