2000 jetta gls

This is normal. The ECM monitors engine temperature and increases idle when it sees the engine is cold. The ECM sets the idle back to normal when engine temperature rises.

You have to make sure the thermostat is working properly and is the correct one. Idle the vehicle for about 20 minutes and then feel the upper radiator hose. It should be hot to the touch. Drive the vehicle and when the temp gauge falls to 'cold' stop and feel it again. If it is much cooler to the touch than before the thermostat is defective or wrong.

Your thermostat may be stuck in the open position. The original thermostat is a 195 degree. Your engine should be warm enough to blow heat in 5 minutes of idling or driving.

Sounds like you got a bad thermostat.

1) Is there enough coolant in the radiator?
2) Have you noticed if the electric fans are kicking on when the temp is going high? I'm guessing that they are not, the temp switch may be faulty.

Have you checked the coolant level? If it is low, what you describe will happen, the engine will run hot, and the heat won't seem to work (coolant level too low to circulate through the heater core). Hope it's that simple, if not, let me know and I'll attempt to help further. Good luck!

The thermostat is starting to stick .
Replace the thermostat.

check coolant level ,must be low ,chck for coolant loss

vacuum to blend doors cancels when engine uses it upon acceleration... there is a check valve that could have failed there at vacuum lines to blend doors .good luck

The first thing i would do is replace thermostat and bleed cooling system, then re-check, It sounds like you have a defective thermostat.