backlash1

Hi
Both of you guys, your problem is more than likely a vacuum leak. The default position of the air flow system when there's no vacuum is 'windscreen' (or defrost).
So, David, since you pull the dash and the heater box out before, it's possible that one of the connectors has come off or (hope it's not this) you kinked or punctured one of the vacuum lines.
Bigronnie, you must have a slow or partial leak, since you're able to use the 'face' position at least sometimes. What happens in your case is that when the engine is at idle (highest vacuum) it will probably work in most positions but when you accelerate (less engine vacuum) it will go to defrost and everytime you stop it will revert back to whatever position you selected previously.
My suggestion for both cases is try to listen for hissing noises at idle or even immediately after switching the engine off, or visually inspect the vacuum hoses both in the engine bay and inside the cabin, including the connector on the firewall. I've also seen the selector switches fail but these are one in a million, so don't go changing it unless you're sure your vacuum lines are all ok. hope this helps. cheers

marc