When I drive on the freeway at 65+ mph I'm all over the road in the middle lanes. My suspension is in good shape and my alignment is great so I know it's not that. In fact on a smooth road it's quite easy to control at 100+ mph. It's just these stupid grooves in the road on the freeway I'm pretty sure are throwing me around. I think they are from wear. Anyone else have this issue? Is there something I can do to make it more stable in these lanes? Changing lanes is extremely frightening as it will lurch in and out of the grooves.

Sounds like the toe in the rear is out, or you have a worn component, possibly wheel bearings or bushings or loose bolts. Try checking for play in the wheel with the car jacked up. But it will most likely be an issue with rear toe.

X2 for rear suspension problems. Check your tie rods.

First time I drove to Florida the back of my 87GT started wagging. Scared the hell out of me. Turns out I had too much toe in the rear and the grooves in the road only compounded the problem. I'd also suggest you do a nut and bolt check including the cradle bolts. Also, tire width can be an issue. My 2005 Evo was fine with the stock tires but when I put on my wide rubber, it would bounce from groove edge to groove edge on some roads.

The tires are wider than stock but I just noticed it sitting on the flatbed the rears look like they have more camber than the fronts. I always just assumed it was because the grooves were farther apart than the tires. The best way I can describe it is like trying to stay in the middle of the wake when a boat is towing you. When the boat turns and you slip outside the wake all hell breaks loose

Check the toe and the thrust angle.

Have the rear checked again.

quote Originally posted by DIY_Stu: Have the rear checked again.

Yup. Rear toe should be 0 - Zero degrees
http://home.comcast.net/~fi...e/FieroAlignment.pdf

I had an alignment done on my 87 GT a couple of weeks ago. NEW alignment machines still have the WRONG specs for rear toe.
They have never corrected them to the specs in the Technical Service Bulletin - TSB 87-3-10 dated March/1987

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[This message has been edited by fierosound (edited 07-14-2013).]