Hi, i would definitely check the stat first, could be jammed shut.Or have you tried swapping the expansion tank cap,if the system isn't pressurising this would cause over heating. Have you any other symptoms, do you lose any water. when the over heating occurs, can you hear the electric fans coming in to cool the radiator. As for you timing belt & water pump price, make sure he puts a timing belt kit on, this includes the belt & the pulleys etc(cost approx