This is just a guess... and it may be TOTALLY off the mark, but something like this happened to me when I went to pick my VX up after having the SC installed (the first time), no check engine light or anything.

They told me the VX was ready, and we took it for a test drive. I noticed right away that it wasn't getting much more power (nothing like I thought it should). As we took an off ramp and were coming to a light, the engine began idling strangely, reving then backing down. Even with the brake pedal depressed it was hard to keep from bumping the car in front of me. We went back to the shop.

After about an hour of fiddling, they told me it was ready. They said they had forgotten to put a plug on a hole on the SC. I guess some sort of vaccum plug. We went for the test ride, and I haven't had a problem since.

Today I was changing my oil and looked at the SC a bit. I felt around, behind the throttle and cruise control cable, and I felt a small nipple tube with a cap on it. I'm not sure if this is the one that had the missing cap, but it would be an easy fix if this was your problem!