keith its so difficult to do this remotely
heres some thoughts-perhaps of no use , perhaps otherwise.....
first and most importantly-does it run smoothly w/o the rods?
if so then it may well be quartering
of course if it runs with rods well, but only for a bit of time, then the quartering would seem in order and as neil observes, something is slipping a tooth
if you are getting ths skipping regardless of removing the rods, consider this
i had a rather heavily used loco-2076- which had the same directional issues-good one way and skipped the other
what in fact was the issue was the axel holes in the gear box had worn, allowing for axel slop-and caused tooth skipping under even modest load or only loco -but this was a normal double worm gear LGB gear box- but the principal may be the same with the gear belt- if this is the case-its a big problem as you will need some sort of sleeve for the axel to sit in...beyond my skills
also
i would not worry about oil on the belt so long as it is plastic compatible-(i actually add LGB and teflon grease on mine as it makes them a bit quieter-in fact as they are delrin-they need no lube-there has been no issue in many years of use )
however grease is best on the worm as it will cling at higher revs, where oil doesnt
if it is indeed the lack of a thrust bearing
as for the ball bearing-while a proper replacement is best-
there is nothing wrong with using a bit of some alternative shim-what might work you ask???-
using goggles and protective cautions-i have used razor blades, cut with rail nipping sheers-the metal is very hard and very slick-if you can get a decent cut-its superb
you will need a tiny sliver-if this puts you off-perhaps some very thin steel strip from the hobby store?? or even brass-obviously with a touch of grease
i am thinking that what might also be causing the issue is, somehow, as neil mentioned, there is an issue of proper tension, either via the belt-ie tight one direction and loose enough to slip another-or, the motor slipping against the worm (which should not affect anything it would seem, as there is only a a single worm and everything else is connected via rods or belt-so the slip, it would seem must be in belt and or belt driven axel)