I’ve had at least 2 cars in my history where something similar has happened. One was a Cavalier that was going through O2 sensors like popcorn, dealer was just replacing them until on the 3rd go-around they found a leaking head gasket. Once they fixed that all was well – luckily it was all under warranty. The other car had a crankshaft sensor that was on it’s way out, took it in twice and they could not find anything wrong. Taking it home after the 2nd visit, less than a few blocks from the dealer, car stalls and wouldn’t not start. They came and got it, replaced the sensor and all was well.
Then again I’ve had dealers do a fantastic job on other stuff.
BTW – not to change subjects but we have been looking at getting an Equinox. Do you really get 32MPG out of it? Anything you particularly like or don’t like about the vehicle?