Volkswagen Repair: 1998 VW Golf - problems with heater, heater valve, golf problems

Question
I am having a problem with the heater on my 1998 VW Golf. It takes a good 10-15 minutes after the engine has heated up for hot air to start coming out. When the heat does start working it isn't as hot as it should be.

Also, once the heat is working, it goes back to cold air when i'm stopped at a light or a stop sign and comes back once I start rolling again.

The fan seems to be working fine.

Any idea what the problem could be ?  

Answer
First make sure that the control to the coolant release valve in the engine compartment near the steering column on 1 heater hose is moving.
  I had troubles like that and traced it back to the thermostat.  Replaced the thermostat(which is on the bottom) filled the car correctly --which is first take the upper rad hose off and pour coolant in the hose until it is full(fills the engine side of the thermostat), reconnect the hose, and pour the rest in the reservoir.  If you try to do it from the reservoir only  the engine side of the thermostat has an air pocket and will never open no matter how long it is running, the engine just overheats.
 If that doesn't solve your problem then I would say the heater valve (mind you mine are over 20 year old with no problems)