I have the fotga DP500 2 QR, which is very similar to yours, with fotga rails and base, and have had similar problems with my gh2 due to the following;
The locking nob on the quick release had to be tightened with pliers (not too firmly) a few times to help it seat itself properly on the rails for a tight fit. The same thing with the locking nob underneath that tightens the slide arrangemnt, and I also rubbed the rails lightly with emery paper.
Make sure the adjustable gear belt isn't too tight around the lens as this changes the gear pitch enough to cause grief.
When I use MD lenses I have had to make adjustments to the md to m/43 adapters as they were too sloppy and moved around a lot, and if the lens breathed it exaggerated the situation, and the gh2 mount is a bit sloppy anyway.
The first time I put a matte box on I had to raise the base a bit, and by raising the lens I started getting better results.
I presume you're using a lens adapter for the 50mm on the g6, so that's the first thing i'd be looking at for play.