Overheating problem

Hi, I have a 2000 Daewoo Nubira, and it has an overheating problem. It
overheats on the freeway, after about 10-15 miles of driving at sustained
freeway speed. When exiting the freeway, the gauge creeps up another couple
of notches when I come to a full stop, then cools down to 1/2 way on the
gauge when I start back up again (never goes down to where it should be
though - half way is the coolest it gets). This is what I've done to date to
solve the problem

* Installed a new radiator
* Installed a new thermostat
* Installed a new waterpump (and while it was disassembled I also had
installed a new timing belt)
* Two separate pressure tests
* PH Balance test (whatever that is)
* Verified that no steam is venting from the exhaust pipe

I've spent 1100.00 so far, and I'm still at square one. Car still overheats.
It doesn't quite peg the temperature gauge, but it creeps up to near 2/3 to
3/4 of max after driving for 20 minutes or so.

Any suggestions would be most helpful. I've combed the 'net looking for ideas,
but everything I've read so far are things it looks like I've already had
done.

Thanks kindly,

Matt