[I didn’t find any attachment from your message, is it missing accidentally?]
I’ve had exactly the same problem, almost since day 1. Later examination revealed, that the slipping took place always in the same sector of the drive gear. The problem went worse and worse and finally the print was like in attached picture. The motor axles seem to be straight. It turns to be so, that tightening the free wheel load spring seems to help at least a bit. I today replaced the drive gears, new ones felt much sharper teethed than the originals (probably worn out because of continuous slipping).
In general, all FDMs appear to live on the edge, what comes to drive gear friction and it’s opposite force - filament feeding system friction (reel rotation friction, from reel to drive gear friction, bowden tube friction and biggest resistance of all - the hot end friction). Even a smallest slip can cause the filament diameter to grow (it looses the round shape) and the small extra friction will flip the balance and the slip will be continuous. Initial slip also quite often leaved filament residue in between the gear teeth, which in turn is source for ever growing slippage. Dual gear systems would probably eliminate this, but they are rare.