After dealer changed my rear crossmember without informing, the rear of my car was everywhere. On bad roads or a filled pot hole that had a bump; when my car went over the rear felt like it moved from side to side, really quickly. Its like it looses traction and the other side catches or grips. Well i went back to the dealer informed them about this and they aligned my car which they should of done in the 1st place. I thought everything was going to better; but no the problem was still there. I had them perform again with another tech. Basically total of 2 times and a full thorough inspection while i was there with the mechanics, and nothing felt loose or not bolted up.
I was very frustrated, so i head down to a local well known tire shop for inspection. The alignment guy said he was booked for 2 days, so i had to come back. This other mechanics checked the car and stuff and informed my front right control arm maybe leaking the ball joint ( 2 months prior a different dealer had replaced that part) so immediately went down to the "control arm" dealer, payed 160 for inspection and the next day they informed me, my front control arm is ok. But my rear bearings are shot, and it will progress. I was shocked and happy at the same time that they found the problem without even mentioning them about the rear suspension problem. So i told ill see you guys soon. Came to the alignment shop, the alignment wiz drove for 5 mins and came back and informed me that they fucked up your alignment. He performed a alignment, and he started laughing he said everything they've done, and turned the bolts were basically incorrect. There was 3 mechanics under my car viewing and amazed. So basically alignment solved my problem, also they checked the rear bearings they said its fine, i told them there was a little play per dealer, they said that was normal. My guess was the dealer alignment machine needed to be calibrated or something.
Also when my car was acting up (i.e. side to side) no lights would come on, no vdc nothing. Also it did not happen every second of the day only on particular rodes and sometimes it would completely stop.
This might be your issue if not, it sounds like a control arm problem, hopefully you can get that fixed.
