[Vwdiesel] Quantum TD overheating, new thermostat - no change

[JoErnest at aol.com]

Hi, 
I have an 85 Quantum TD which appears to be overheating:
Temp needle reaches half way between LED and top white line, and would keep 
climbing if I don't turn on heater.
Top radiator hose is hot, bottom hose is warm - easy to hold in hand.
Jet of hot water spraying into expansion tank (means water pump is working?)
Forcing water into radiator - gushes out the other hose.
Forcing water into exp. tank, water gushes out the top radiator hose.

I've replaced the thermostat - no change.

Any ideas? 

Thanks...............JoE