1. It could be the engine cooling system needs flushing.
2. Or your heater hot/cold valve is not opening properly.
1. Buy cooling system cleaner, and what some forget, ANTI FREEZE. to replace lost on draining. Follow instruction on cooling system cleaner packet/bottle. During final re-fill add the Anti freeze to the required/suggested strength.
2. If you haven't got a workshop manual go to/phone you local library and ask if they stock workshop manuals for vehicles, ours did, the fault could be the heat/cold control valve on the heater unit and it's a remove it job.
Rob
October 2005