My 1992 Toyota Pick-up, 22r, the idle flucuates up and down, up and down at a rapid pace. This occurs while sitting idle and slows down a little when the brake pedal is pressed. While the idle is fluctuating there is no heat to the vehicle. After driving for a period it will smooth out and idle normal, vehicle heat works normal, but it will start doing it again after driving for a period.

. Replace coil.

. Clean Idle Air Control Valve/hoses, throttle plate, ports, and throttle body:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-46563-7.html#post1054149

Also try cleaning the MAF sensor. The Mass Air Flow Sensor regulates the air mixture that goes into the throttle, and if dirty or defective will cause poor idling and bad gas mileage. It is located off the air filter between that and the throttle.You have to take off the large hose that is attached to it. You can use electronic parts spray to clean it. There is a small hole in the top of the large round metal piece. This hole is what goes to the sensor, which is the small box on top. Don't try to open the sensor itself! Just spray the cleaner into the hole. Let it dry for at least 1 hr.
It may be that the sensor has gone completely bad. You might want to have it tested. If you don't know how, a mechanic shouldn't charge you very much.