trebor

I have a new radiator, replaced thermostat twice (thinking 1st one was sticking), fans are working. No evidence of head gasket leakage. Will run fine for 8 to 15 miles; temp will start rising- if on a road where I can floor it, temp will lower to normal. If I have to pull over, shut off and leave fans on, coolant bubbling in reservoir. Does not happen when idling. Pretty sure water pump was replaced w/ timing belt. Any ideas?