I took my 03 Passat V6 automatic (98k) in to get one of the Catalytic converters replaced, as it was leaking and the check engine light was on. I’ve used this shop before and they seemed to be competent.
But when I drove the car away, something kept grabbing the car, like the transmission downshifted (although it didn’t). The entire car jerked or lurched strongly. This occurred every few minutes.
I drove it back, slowly. The shop had no idea. They “reseated all the connectors that they went near” and that fixed the problem, apparently. Been OK for a few weeks now. They had no idea what happened or what they did to fix it.
I had noticed similar occurrences every since I owned the car, but only about once a year or so. My guess at the time is that the transmission got confused, for example, if I stepped on the gas just as it was upshifting, so it had to do several shifts together, or something like that. But a similar lurch. Of course the dealer could never find anything.