And one more thing: all this misalignments does not explains the phenomea I have had last time (and many times before): after having a good tracking something goes wrong and starts to drift in a direction. The direction and drift speed seems to be random, and changing in time.
I think having the scope not perfectly level or otherwise badly aligned should cause a constant drift at a given target on the sky, at least for a while (which 'while' is measured rather in hours then minutes...)
No explanation.