I don't want to harp on this but several folks have suggested checking your coolant concentration. I know, you've replaced the coolant with 50/50, but if you haven't done so, I would strongly recommend actually checking the concentration with a tester (like the Prestone one described in JimF's post). I know from personal experience that it is virtually impossible to really drain all the coolant, and so if the previous ratio was high and you add a 50/50 mixture it'll STILL be too high. The only way to know is to test.
I would definitely start here before working on more esoteric possibilities.