Vibration issue, tire related?

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I've been slowly but surely over the years chasing down every single little vibration issue on my car to make it as smooth as possible at highway speeds and I've got it pretty dern good now...but I'm still fighting one thing and I'm stumped over what it can be.

So I'm posting this question to see if anyone else has an issue like this and if it is common or unusual in my case.

When I take my car out of the garage and go down the highway, initially it'll have the typical flat spot feel from the tires sitting in one spot. It generally only takes about 3 or 4 miles of highway time for that to go away though...then the car is super smooth wherever I go with it. There is virtually no vibration at any speed when the car starts out cold like that.

But say I drive it to work, and it sits in the sun all day...on the trip home it'll have a shudder that feels like it's coming from all 4 tires between 60 and 70 mph. I can drive through it, or raising or lowering the speed just a bit will make it come or go, but it'll settle back in and not go away. It's not bad enough to make the rear view mirror unusable, but it is certainly noticeable. I first started noticing it on the way home from an autocross event, it scared me pretty good the first time because I thought I broke something in the rear end. But I drove the car again a couple of days later and she was smooth as silk again. The shudder only happens after the car has sat outside in the heat for a while after the tires have been heated up.

It is definitely tire or drivetrain related, pushing the clutch in and coasting doesn't change the vibration at all. and I feel it in the steering wheel and my butt, so it feels like it's comes from the front and the rear. Is this just a trait of 200 tread wear performance tires? Do the rest of you that run a high end sticky street tire have this happen as well?

All of my wheel bearings are fresh and tight, the Falken 615Ks have about 750 miles or so on them, everything else on the suspension is tight and fresh and even the rear axle and clutch have all been freshly rebuilt with no change in this attribute...it almost HAS to be the tires themselves right?

And I'm okay with it...as long as I know that's what it is. If there is something I can do to help it I'm in, or I can just deal with it.