I have a 92 Honda Civic. One day the temperature shot up to really
high on the gauge. I managed to get home etc. without damage to engine
after allowing to cool etc.

I inspected the next day and replaced the radiator and coolant. Took
it for a test run and worked well... but then shot up to the red
again.

I have recently had the water pump replaced and am sure the fan is
working properly.

I know... your thinking thermostat right? Well here is the catch.
The temperature goes down when you put it on full blast??? This makes
for uncomfortable driving during the summer months!

Could this be the thermostat or a blocked heater hose?

Any ideas would be appreciated.

Gerard