Q: Temperature guage reading high when idle

After driving my van for a while.... while idling at a stop it will start to go high on the temperature gauge. If I turn it off at this point it hesitates to start. I have had a mechanic out.. he claims to have checked the thermostat with heat and cold water and it is working fine. and he said the fan is working fine. The mechanic said he drained the radiator fluid and refilled it ( i think he called it bleeding ) and when he was done it seemed to work fine again, we had success, that there might have just been air in the system. according to him..... Wel not but a day or two later it began doing it again. I have checked the reserve and the water was empty but we never notice a leak.... this is actually the second time since this problem began that it was empty.... the mechanic is telling me it MAY be the water pump and it will be a $600 repair because of how difficult it is to get to in labor.. and then he still cannot guarentee that is the problem.... any advice??? Please HELP...

My car has 150000 miles.
My car has an automatic transmission.