Just thought I'd throw it out there...
It seems like the next step is to put it on a Tech2 and read all of the detailed diagnostic information. Symptoms that severe that "come and go" seem like they should have clues that reveal themselves on the scanner...
I don't disagree with you, but being a former service writer I know the routine.... NPF - No Problem Found.
Here's what will happen:
7am - Drop car at dealership and tell them just to "hook it up to the diag machine".
2:30pm - Tech goes out to vehicle, starts car with diag tool hooked up.
2:35pm - No code, no issues found, takes for a test drive
2:38pm - NPF
2:50pm - tells writer there is no issue and NPF
5:30pm - writer calls me and says NPF
5:50pm - I pay $185 for a diag that did nothing.
2:35pm - No code, tries to look at parameters and realizes that vehicle is not standard code in PCM
2:38pm - Rewrites PCM to original GM tune
2:50pm - tells writer he had to flash PCM
5:30pm - writer calls me and says we reflashed your PCM
5:50pm - I pay $185 for a stock tune and vibration still persists
Sorry to anyone that's a writer, but this has been my experience in the dealership world at several dealers now...