The 2.0L almost never sees head gasket issues, so be fore condemning them I would look at a few other options.
Were there overheating issues before replacing the t-stat? What were they?
Things that could contribute to overheating:
-Air pocket in cooling system, as the system was just played with this would be a good place to start. Rebleed/burp the system to get the air out.
-plugged radiator, as mentioned above.
-fans not coming on, do they cycle?
-bad water pump? Also very rare.