[JoeFromPA]

Jhandy - I think that's a really good description of some of the symptoms I experience, where I can't always consistently reproduce the imbalance at a given speed. The only times it ALWAYS appears it at 81+ mph.

Otherwise, I've seen it migrate around 45mph on and off.

I'm going to rotate to my winter tire setup. If it cures it or it changes significantly, then it's definitely tire/wheel related and I can at least narrow it down to there.

If not, then I'm going to suspect a worn out bushing or wheel bearing allowing too much slop. The wheel bearings themselves make no discernable noise, so I'm disinclined to think it's wheel bearings. Alignments can cause imbalances, but rarely ones that wander in and out of vibration. Still a possibility though.

Worn out control arm bushings or bent control arms can cause all sorts of issues.

One thing I will always remember though is that lightweight, short-wheelbase cars with rigid bushings and suspensions setup for instant response....

Well, those cars tend to magnify even the slightest issues. So a hairline crack in a bushing or imbalance somewhere will magnify alot more than it would elsewhere.