Tires and Alignment

[Pastor T]

I purchased my 2000 base Boxster 14 months ago. The back tires were OK and the fronts were shot due to bad alignment. I have been buying tires at Big-O for years and the guys in my local shop are friends. So I went there. I had it aligned and new tires on the front. Left the backs alone. (It had Falkens all around) They recommended Nito Motivo 205/50ZR17's for the front. After 13 months and 14K miles of combined city and highway, I was very happy with the handling and wear on the fronts. I am a "Spirited" driver and never waste a chance to take a corner or on/off ramp at the limit. My Boxster was stuck like glue. I was a happy camper. Now for the bad news. By now the rears were shot and so back to Big-O for alignment and a set of Nito Motivo 255/40ZR17's for the rear. After a couple of corners I was literally scared. I thought the back end was coming out from under me. Those ramps that I was taking at 80 were now down to 50 and I was loosing it. I went back and they told me it would be like that for a while due to "tread squwish". So I forced myself to slow down and hold on. I checked my pressures and made sure I was at factory specs 36 rear and 29 front. Even bought a new gauge. Same handling. Now 2 months and a couple of thousand mile later, same thing. The question is, "Where is my car and when can I have it back". I need help.