Baffling Problem

'96 Cutlass, 3.1: Engine runs well at all speeds, including idle, but one very puzzling problem. When I'm sitting at a light and start to release the brake pedal, and the car just starts to creep forward, the engine(?) chugs, sometimes very strongly. As soon as I release the brake, it's gone.
It's baffled a few mechanics. I've been working on cars for 30+ years and never encountered this. Is it a vacuum problem the brake booster?

The first thing that came to my mind was a vacuum problem with the brake booster as you suggest. You may either have a leak in the booster itself or it may be in the line leading from the engine to the booster. Change the vacuum line first, a lot cheaper than a new brake booster. If that doesn't correct the problem then the booster will have to go.