1998 Ford Escort overheating

I have a 1998 ford escort. About 7 mo. Ago I blew a head gaskst.I got it fixed the tech did a good job. But I nocticed that it would not hold coolant for along period of time. The car drove fine. Resently the car over heated I shut it off and coasted down the hill to avoid further damage.I check the raidiator for pressure it was fine as well as the cap then I found a leak from the waterpump so I replaced it. The lower raidiator hose has a slite leak because I did not put it on the water pump flush. But the car sounds and runs great. But if I drive or let it idle for more that 10-15 min the temp shoots up and it overheats I think it the thermostate what do you think