Haven't had this happen while I was driving the car, but my wife tells me that sometimes when she is stopped at a light, the brake pedal goes soft just for an instance, depresses a fraction of an inch (no more than 1/4 of an inch), and then gets hard again. She says it happens intermittently and only when the air conditioner is on. She also notices that the engine idle speed seems to change at the same instant the brake pedal goes soft - the idle speed drops. Sounds like a manifold vacuum / brake booster problem. It just started happening recently, and it doesn't seem to have any effect upon the braking capabilities of the car nor the pedal pressure required. Any ideas?

Steve T, 94' 850 GTAS, 90' 740 wagon