engine races and car stalls out when stopped on 2000 Toyota Camry

It happens sporadically, but yesterday i zeroed in and noticed that when the fan is on and the the defrost position is chosen and i come to a red light the engine races and sort of bucks. I tested it in other settings and it only happened with the fan on and the defrost setting???? I recently had a new thermostat put in and recently a new water pump.

by in Northvale, NJ on January 13, 2013
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1 answer
ANSWER by , January 13, 2013
take it back to whoever did the water pump