Someone on the A4 listserve at Sailnet has suggested spraying automatic tranmission fluid into the intake manifold when the engine is running to free sticky valves. It's worth a try. I don't see what harm it could do. I would probably also try spraying some kind of penetrating lubricant around the valve stems thru the spark plug holes each time I shut it down for a few weeks.
If the engine was recently rebuilt and had new valve guides installed there may be no solution except for removing the head and valves and reaming out the guides.
There is a retrofit valve spring kit that is available that puts a stronger spring on the valve. This would require removing the manifold to access the valve springs.