2005 chevy impala running hot

I have had this car for four years. The
second year I had to have the engine rework
because it blowed a head gasket. Now two
years later it is running hot only in the
evening. I have put two thermostats and a
water pump on it. What else could it be? My
mechanics say that it has blown a head
gasket again but there us no white smoke
coming from the exhaust like last time.