Surging 2005 Scion XB

I have a 2005 Scion XB. I love it, but for the past few days it has developed the nasty habit of trying to surge forward when braking. This only happens occasionally and only when the car is cold, but when it happens I have to be ready to tromp on the brake really hard, especially when parking. The car has automatic. The Toyota dealer has not even been able to reproduce the problem yet, much less fix it. Any suggestions on what to look at?

Possibly a bad brake booster. Brakes are several times stronger than engine, but not when you have extra power + weak brake. Just be ready to shove the shifter to neutral.