Also, maybe the OP is forgetting to put in the o-ring? I'm reaching here, so bear with me.
No o-ring will alow the coolant to flow around the t-stat and keep the car cooler than normal. The t-stat is supposed to fit INSIDE of the o-ring as well, if you just drop the o-ring on top of it and bolt it back down you can run across the same issue there too.