It was a sunny day and decided to get my new ANSA muffler installed. I store the car on three hyd. jacks with the tires about 6" off the ground. I pulled the car cover, dropped the car back onto the ground, discounted the battery maintainer and started it up to warm up. All seemed normal at 25 mph while driving out of the subdivision. I pulled onto the main street and when the car reached 35 mph it started to vibrate badly to the point I had thought I blown a tire. I pulled into a parking lot and to my surprise all the tires looked normal. I then checked each wheel to make sure it was not loose. All seemed normal so I decided to continue on my way and as I reached 35 again the vibration started again. I tired speeding up, putting the car in neutral, and steering side to side and nothing seemed to make it better or worse. I drove it about 5 miles and by the time I reached the muffler shop the vibration was all but gone. I drove the car home on the expressway with no problems and by the time I was home everything was back to normal. I spoke with Gary (vaulk) and the only thing we could come up with is maybe the weight of the rear end was placing a stress on the CV joints and caused this. My question is should I just leave it on the ground for storage. I'm also worried about the drive down to the dragon and having problems. I think it would be a waste of time and money having it looked at since the vibration is gone. Any suggestions would be much appreciated.