Volkswagen Repair: heater problem, volkswagen passat, expansion tank

Question
I have a 1990 Volkswagen Passat.
My problem is with the heater which works well for the first 20 mins or so of driving, but after this starts to blow cold air.
Is this a thermostat fault?
The expansion tank is filled with coolant to the correct level.
Can u help? Thanks a lot.  

Answer
That is where I would start.  Also check the black shut off valve located on the firewall near the steering (on one of the heater hoses to see the action of the lever.)Might not be fully opening.  Thermostats have been a sore spot.  A remote chance that the heater core itself is plugged up, but if you have been keeping your antifreeze fairly clean, I would look there last.  Better to change the thermostat, and get the type that are fail safe(in otherwords, should they fail, they fail open(not closed)like you may have right now.