300 Tdi heating problem

My 97 ES will not blow hot air through the internal vents unless you are barrelling along on the motorway or a fast road. The engine heats up ok and the guage sits alright but even after getting through the vents on a good run, when you are stopped in traffic, after a while they go back to blowing cold air?

Please say it is an easy fix!

Any body??

I am beginning to sound desperate now!

Sounds like your heater matrix is blocked, if you look behind the bulk head you will see an in and out pipe,undo and try and flush it out with a hose pipe/buy a chemical radiator flushing thing!

Sounds like an air lock to me.