95 GA heating/cooling anomaly

I have a 95 grand am 2.3 Quad 4 dohc with 136,000 miles. It started running slightly hotter than usual, around 220, and I discovered that the head gasket was leaking down the back of the block. I replaced the head gasket, and it's running fine, except that it is overheating...temp gauge just pegs out. The head was milled, checked for leaks, the radiator flushed, the heater flushed, new hoses, new thermostat, new water pump, new sending units, and a switch installed to manually start the fan. I even milled the water pump housing to tighten up the clearance between the housing and the impeller. When I drive the car, the intake side of the radiator gets slightly warm, and the discharge side is ice cold...the gauge is pegged at 280, yet the radiator discharge is cold, so I know that the thermostat is not opening with that cold water on it. I drilled some holes in the thermostat to allow some circulation, and this has fixed the problem, although it now runs a little cool (I'll probaby install a new thermostat with fewer holes drilled) but I don't understand why this wasn't a problem before??? Possibly the poor performance of the old water pump caused it to run hot enough to allow the themostat to open, but I don't think the car was designed that way.
Any tips would be great...