The heater on your 1986 Celica GTS 5 speed 2.0 only works sometimes what is wrong?

Here are a couple of basic things to check: 1-Make sure the coolant level is correct in your radiator and overflow reservoir. 2-Make certain the thermostat (in the water outlet housing) is in good working order. This should be replaced every 24 months because it is cheap/easy insurance that your car's cooling system will run well. Also, make sure that the thermostat in the car is the correct one (with regard to temp) stated in the manual. 3-Are the hoses leading to the heater core (in the engine bay) swollen/damaged/old? 4-Is the actuator arm/servo/solenoid for the heater core bypass functioning correctly? Basically, when you "turn on" the heater, something has to physically open/close the coolant bypass valve to the heater core to allow warm/hot coolant to flow into the heater core (under your dashboard). If this process is electronically controlled, it is possible that it is failing to open that valve some times. If it is a mechanically controlled process, the heater bypass may be obstructed. 5-Heater core may need replacing. If the heater core has a lot of build-up in it, the coolant will not circulate as easily as it should which can cause the heater to not function properly. There is probably more to check, but its nearly 3am and Im not thinking clear enough to proceed any further. Good luck.