Could be a number of things, resistor card could need changing (under the glove box and its a gold credit card, maybe), coolent could do with changing (most likely), you have a blockage (not that likely)
Other thread has been locked, try not to post two thread on the same topic in the future please.
i had this on mine. changed the coolant and flushed out the heater matrix. just disconnect the two holes that go into the bulkhead , them i used two old pipes, one to drain water away from every thing and the other i forced a hose pipe into it and blasted it with water til it comes out the outher one clear. then re-build and fill with coolant.