Yeah, definitely a vacuum issue. I have a very small leak somewhere, such that only once in a great while (a few times a year maybe), if I sit with my foot on the brake for a long time or hit every red light through town it starts to shift harder and I have to let it idle for a few seconds before it'll shut down.

You'll probably just have to start isolating systems with a handheld vac and see what won't hold it.

-Rog