cooling system problem 1998 Cavalier

[drsuess]

I have a quick question to which I hope someone can help me out on...I have a 1998 Cavalier with a 2.2 liter in it. The car has almost 200,000 miles on it and it is still running great, but I do have the following problem. The original radiator was recently replaced with an aftermarket one (The aftermarket one is a little wider.I think it might be bigger core wise, but is an direct replacement according to autozone) Anyways, I replaced it, along with a new thermostat (Factory 180 temp) Everything is fine except the car does not heat up at all when driving..when the car is stopped at a stoplight, or idling without airflow going through the radiator the car heats up fine. It seems like the system becomes unpressurized when driving. I have noticed that thee is a small, small leak on the waterpump, which I have yet to fix but wouldn't think that would cause the problem that I am having.
I will say that the car initially did heat up temp wise when the original radiator hole happened(To what extent is unknown) A woman was driving it when she noticed the BIG red dummy light.
Anyways, I tried to replace the thermostat again (Thinking the first one was faulty) but still no difference. Any suggestions/comments are appreciated.
Thanks!
NS