2007 lexus es 350 overheating when under a load from a/c or when driving

After sitting for about 2 hours with the vehicle running i noticed the needle
rising to the "H" when i began driving it cooled down but when i sat still it will
begin to get hot again. We just bought this car unfortunately at 236k miles
and were told its mostly highway miles which is fine. Could it be that water
was being used for a long period of time and destroyed the water pump
impellers i'm not sure what else it can be without taking it apart. i Don't think
its the thermostat and so far no signs of a blown head gasket also both fans
seems to be blowing but im not sure if they are blowing strong enough.