Mike --

Several times on different cars, after replacing a thermostat or water pump, I've had the engine overheat slightly. I've decided there is a tendency for the cooling system to become airbound -- to form an air pocket in the water pump, preventing the impeller from moving the antifreeze.

Keeping the heater at max temp (allowing flow through the core) and going through several warm-up cycles solved it for me. Also, carefully (don't get burned!!) allowing the "head steam" to vent from the overflow tank during overheat has helped.

Remember to carry an extra gallon of coolant during your first several road tests.