I have a 94 Es 300 and the problem that I have been having is that when I drive for longer the 30 minutes, the antifreeze in the surplus tank will increase until it overflows, it does not seem to be circulating properly. The antifreeze is at the correct level before driving, after driving the antifreeze would increase in the surplus tank depending upon how long i drive, then it would it would go down after the engine has cooled. It is not overheating at this time but if i drive too long it overflows through the surplus tank, thus less antifreeze in the engine

What can it be, one mechanic says it could be the thermostat if not the head gasket??? any insight.....?