Well 1st things first.
1. Was cooling system rusty when u first bought the car? Was there any signs of rust at the radiator cap? Inside the expansion tank?
2. Does the water/coolant level went low and needs topping up every time? there may be leak. Conduct pressure test to find out. there may be leak at the radiator hoses, water pump and even at the radiator itself...either the core fins or at the top/bottom casing.
3. Is the cooling fan operating efficiently? revs up the engine at 3,000rpm for a few minutes and see whether temperature increase or not..and whether the cooling fan operates or not. the fan may malfunction due to worn out motor, or defective temperature switch or just simple wiring problem including faulty relay or burnt fusible link.
4. If the radiator looks contaminated with rust inside, there are high chances that it is clogged even though water flow is smooth. out of 30 cores for example if 12 cores are clogged water may still flow but the cooled coolant may not reach the engine in circulation. send radiator to specialist for servicing..this may cost you between RM30-40.
5. do you feel like ur car doesnt have enuff power during acceleration or hill climbing? if its true then possible cylinder head gasket had blown. the easiest way to check is remove radiator cap (only when cold) then crank the engine. if water is blown out from the radiator mouth then it means new head gasket is needed.
gud luck to you..
p/s maybe need to replace the car but i'm juzkidding...