Audi Quattro Minimum Oil Warning Light

I have a 2001 Audi A4 3.0 Quattro,and for some strange reason the minimum oil light keeps coming on. There is plently of oil in the engine because it is the first thing I check, but the very strange thing is once you take out the dipstick or open the oil filler cap the warning light goes out. I have replaced the oil sensor in the sump and had a engine flush which did help for abit but now it has started again.
Is it a electronic problem? or could it be a hose blockage somewhere.

Jag