Interesting that this same possible problem, low shift lever friction, surfaced in another thread recently. AS in that thread, TimmyT has a simple experiment to try to see if der adjustment or cable/lever holding are part of the problem. Remove the der and place the chain on the middle ring then test ride. Reinstall the der and leave the cable off but wind in the limit screw so the cage is clear of the chain while on the middle ring and again test ride.

I have seen worn chains and rings do this auto shifting thing when under power. Replacing both (and often the cassette too) has fixed it before. Do I have a pat explanation? No but it has worked. Andy.