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.

Asked by for the 2000 Toyota Camry
take it back to whoever did the water pump