I changed the complete themostat yesterday mate on mine as when I took it off it had swollen the rubber o ring and cracked a bit of the inside off. A mate of mine who's a mechanic told me not to bother with just changing one part of the case as from experience, a few months down the line, the other part will go or it will crack again so just replace it all, straight from for, no messing about.

Take it off and examine first. If it's ONLY a seal or gasket with no signs of a crack then I'd just replace that seal or gasket. Any cracks or defects, replace the entire stat. 120 degrees doesn't sound like the fans are kicking in mate. In traffic, I can normally tell when my fans come on or are on.