Steve,
Make sure that you don't have any air in the system. If your rad. sits lower than the rest of the system, that could be the prob. Try jacking up the front of the car, or parking on a steep hill and let it run with the cap off, topping it up as necessary.
Also, check that the thermostat is working. I've had new ones that didn't. If you have a thermometer, try boiling some water, with the thermostat in it and see what temp it actually starts opening. Back off your timing a bit. I've found that 8 or 9 degrees works well for sea level.
Check your plugs, make sure it isn't running too lean.
Last thing I can think of, check that the bottom hose isn't collapsing (should have a spring inside, or something).
Tim