Partially Solved Occasional Layer slip

A small amount, random.
I need to go through the gantry and make sure everything is tight as well as the belts.

It is slipping about 1 cog worth at random spots and not every print (yet)

 

It will be interesting to find out the cause of the problem. How about an image showing the "slip" problem?

 

Let me see if I can get one. It is a sudden (but small) complete layer shift in one axis (X I think, will need to check that as well). I am sure it is a loose belt or drive cog, but it may be the gantry misaligned... I hope to have time later today to get in there and do a tear-down.