I have had this happening to my previous Golf 1.4 IV from 1999, time and again. It drove me crazy, and I noticed I am not the only one. The problem in my case was that, in the engine bay, there is a multi-connector plug just underneath the windscreen (water/damp gets in somehow). The metallic points of the plug corroded, hence problematic communication between ECU and rest of car. In essence easy fix as points need to be cleaned but takes lots of time to get to (windscreen wipers have to be removed etc. etc.), a mechanic with good ideas might be able to help.
Hope this helps,
KS @ clickmechanic.com