Heater comes on and go off when it wants to.

What you're likely referring to is the blowing air starts and stops erratically. Since the heater uses heat circulated from the engine, if the air is constant and the heat comes in and out then that's a different issue.
Firstly, if the airflow starts and stops, you'll find a loose connection between the the fan switch and the blower itself. If the connections are sound, then look for a loose ground connection to the blower. If all electrical connections are good, and it still starts and stops erratcially, then it's time for a replacement blower.
Secondly, If the air is constant and the heat comes in and out, the the options are a faulty water pump or partial blockage in the colling system or heater core. If the water pump is failing and working erratically, then when the heat dissipates in the cabin airflow (it has used all the heat in the heater core and no circulation to replace the cooled water with hot water) the dash temp gauge should start to climb, indicating that the coolant is no longer circulating, the heat has left the heater core but the engine is not being cooled. You'll need to replace the water pump. If this is the case, take care of this ASAP as you run the risk of a an engine overheat.
If the dash temp gauge looks normal, then you're likely looking at a partial obstruction in either the cooling system or the heater core that blocks the flow and then works its way loose erratically. Limescale and crud buildup in your cooling system (especially in a 15-ys-old car) can cause this when flakes of buildup brake loose from the walls of the larger channels and get caught up in the smaller ones. Your local parts store will have additives to clean out the cooling system, but if they don't work (they can only do so much) then you're looking at a professional cooling system flush. If the issue persists after that then you're likely looking at a heater core replacement.
Also check to see that your a/c is not kicking in erratically. It shouldn't completely cool down the full-hot airflow, but it will cool it. You can tell if the a/c unit is kicking in as you'll hear the compressor clutch kick in and the engine revs/sound will change under the load. If this is the case look for a short in the a/c wiring.
Hope this helps, and please rate my answer.