I'd go with what Jackaroo mentioned... check the thermostat and the water pump.

A stuck-up thermostat will usually slow down coolant flow so when the engine is revving/moving, the heat builds up.

One symptom of busted water pump is that you lose water when the engine is running.

Kung medyo luma na pareho at di pa napapalitan, maybe it's time to do so na rin as it's not that expensive for those parts.