I have 2006 VW beetle with the 2.5L engine. Once the engine has warmed to operating temperature, and I am driving it around, the heater works great. As soon as I stop at a red light, or am idling for any length of time the air coming out of the vents begins to lose temperature until it blows cool air. All I have to do to get hot air again is give it a little gas, say 200 more rpms, and the heater immediately responds with warmer air, hot air if I maintain 1000 rpms. I have read related information on here and think that if the heater works great at other times, I do not have a blender door issue, clogged heater core, or low coolant level. The coolant level is fine, by the way. Any help would be greatly appreciated!
Problem solved! The key was to finally locate the source of OBDII code PO 0171, left bank lean, which I realize was not part of my original question. Anyways, I found a leaky air hose connection between the manifold and the MAF, once I corrected it, the engine idle smoothed out and now we have heat as designed at idle.
You may have air trapped in the cooling system. The air needs to be burped out. The method varies by make, model, year, etc. Due to variation between models can you tell us what country the car was made for?