Radiator cap in good condition? Timing properly set? Right heat range spark plugs?

What temp t-stat did you get? I've seen 160F, 180F, and 195F offered for the 4G63. Maybe try going with a cooler one. Maybe you had a 160F or 180F before and now you have a 195F. Most of the parts stores carry the 180F ones, you'd most likely have to order a 160F.

EDIT: Looking at Rockauto.com, they carry 180F and 170F t-stats for your car, and they're around $10.

I dunno man, could be any number of things.