Toyota Repair: 94 toyota pickup the engine reves up and down, air fuel mixture, coolant temp

Question
my pickup is reving up and down when in nutrel and when you driev it it also over heatsall the time

Answer
The overheating problem needs to be addressed first, it is the cause for the idle fluctuations, there is air in the coolant system, this pocket of air causes the coolant temp sensor in the system to read air instead of coolant, the computer sees this as a need to control the air/fuel mixture and which causes the idle to go up and down.
Check the cooling system for an external leak, check the radiator and all the hoses for leaks, these engines, whether 4 or 6 cylinder also had lots of headgasket problems so it may be a good idea to have it checked for that possibility. It could also be a partially restricted radiator or a stuck thermostat, try replacing the thermostat first and see if it helps if there are no leaks found.