Thanks for all the inputs!

Yes, I'm skeptical about tires/balance problem too. Just want to rule out any possibilities with my mismatched old front tires and balance issue (haven't done a balance since I got those tires 5 years ago). If new tires/balance don't fix the problem, will check the subframes first then another alignment with a little front toe-in as suggested.

Mounting new tires later this week, will report back. Thanks for all the help.

 

Just a quick update. I've got the new front tires S-O4 to match the new rears, and wheels balanced. After about 100 miles, the wobbling/unstable/tramlining feel seems to be gone. I've tried 70-80mph on the route that I had trouble with, now the car felt much more stable. I'll continue to keep an eye on it, if it happens frequently again, I will do another alignment.

Thanks again for all the help and input!

 

I was going to mention the mismatched tire suspect, and it looks like that is the culprit.

I agree with Billman, but also the S2k is extremely sensitive to even the slightest mismatch of tires, Lto R, or F to R. Tramlining is the result.

 

Just a quick update after about 1,200 miles. The new S-04 tires broken in nicely, and car feels very stable right now. Like many others have mentioned, the mismatched tire (old fronts w/ new rears) and lost of wheel weights were the main culprits here. Again, I really appreciate all the feedback and help. If the new front tires and balance didn't fix the problem, I was so ready to do another alignment w/ more front toe-in and subframe collars.

On a side note, I did add a CR styled wing on my trunk, to kind of balance out the aero from my CR front lip. On freeway speed, I could feel the car being more stable, with less "uncertain" or "light" feel at the back. My MPG also dropped by about 2 MPG, possibly due to stickier tire and the wing. All in all, the combination seems to be working and the car is feeling more stable at higher speed.