I've had a look at the TCM today to see if there is anything wrong with it. Everything looks fine, nice and dry. Definitely still the original unit. I also had a look underneath the car (as much as its possible with it being on the ground) to see if there is an ATF leak that could cause the slipping gear related codes, but everything is so oily that its impossible to tell where its coming from. I cleared the codes and took for a very short drive shifting through all gears as often as possible, still driving perfectly fine and the only code that came back is the MAF (16486/P0102 "Signal too Low"). I haven't checked the wires properly yet but will do that asap. I need notice this zip-tie around the connector though, is this normal?
I guess that confirms that the gearbox codes above are intermittent. Maybe there's still a chance to the ATF level is just too low?