I've had a mechanic explain to me a common problem with VW automatic transmissions. It's really a questions of poor engineering sadly. From my understanding the the valve body, servo piston, and another part which sadly my brain can't remember are made of 3 different metals. The valve body is the weakest of the metals (aluminium) and over time ends up deforming causing the transmission to poorly handle gear changes. The common indication of this problem being a very sluggish change of gear and high RPM...
From what I gather it's become a common problem with well aged VWs automatics... Your only option is a new tranny :P