Sounds like you have radiator trouble. Have you checked the fluid level in the radiator?
cold. If you open the radiator when the car is hot, it can spray all over the place.
0.976
check your radiator hoses and change gaskets
for the heater just to cut out from hot to cold then kinda heat up again slighty then drop again would be lack of antifreeze and water in the radiator. check that first. . check it... when it the car is cold open the radiator cap and look inside for fluid usually greenish in cool. if no fluid add a 50/50 mix of water and antifreeze to fill. check fluid levels again the next day ....there may be a possible leak.
Open the radiator cap when the engine is cool, check the radiator's fluid level, and add appropriately. NEVER OPEN A RADIATOR CAP WHEN THE ENGINE IS HOT OR OVERHEATING. Check your car's owner's manual for specific details.
You have to check the motor thermostat. Replace it, and you will be warm again.
The heater core is a radiator in the car, warmed with coolant from the engine. When you put the Air-con on hot, and turn the fan on, the air is blown over it to warm the air (and you in the car) up. As it contains radiator fluid, it can cause the level to drop if it leaks. Check the carpets in the car to see if they are wet with radiator fluid, or to see if the car leaves a puddle of fluid coming out of the air con condensation drain.
Check fluid level WHEN IT cools off.
If your car running less then 10 minutes then it's considered a car with cold transmission fluid (fluid should be in the region of cold level marks, but not lower), if more then 10 minutes then it's hot (fluid should be in the region of hot level marks, but not higher). When you check transmission fluid level your car should be running.
It will tell you on the stick, either "check hot" or "check cold", running or not.
check the fins on your radiator they could be corroded and loose decreasing your radiator s ability to cool the engine or the thermostat could be defective also