Voltage could be the issue, since the machine uses different steppers for X/Y and Z/E. Might have switched some drivers around...

Regarding the belt, I'm thinking of making a pulley system that will allow me to tighten the loop belt. I can't seem to find any other way to tighten it, without drilling in to the chassis. The screw holes do not look adjustable at all.

Thanks for the comments, guys. Much appreciated!

layer shifting can be a couple of things - none of which are fixable with firmware modifications.

#1 - loose belts are the number one culprit for layer issues. A closed loop belt should have some way of adjusting - loosen the screws holding the motor in place and try moving it.

#2 - voltage to the motors - you want to run the voltage as low as possible without causing skipping.
when printer is running, touch the motors - if they are too hot to hold your finger on then voltage is most likely too high. Overheating motors will cause shifting like you are seeing.

#3 - if voltage is set per above, but the motor still gets hot, you may need to add a fan to it to keep it cool.

I've been trying to make an old Creatbot DX3 run smoothly for about 6 months now (that's how long it took me to realize that you should buy new filament instead of using 3-year-old, brittle one, that you found in a dusty, moist basement).

The printer works as expected and makes great and accurate prints with even 3-4x the speed I'm using at the moment (1500mm/min), but I keep getting layers shifted! Slowing things down helps, (though no one likes waiting 5h for a small print), but then the oozing gets out of control....

I've attached two print pictures (retraction cubes and a custom one) where both oozing and layer shift can be seen. The material is PLA, using Simplify 3D to slice.

I've checked/did the following things to try and handle the two issues:
- belt tension, y-direction belt is a bit loose (no way to tighten since its a small loop), but the shi(f)t happens in both directions
- jerk adjustment (see code below)
- max acceleration adjustment (see code below)
- max stepper motor current adjustment (see code below)
- replacing stepper motor driver circuit
- increasing the current on the driver circuit (potentiometer adjustment)
- slowing everything down (1500mm/min print speed, 20000mm/min x/y movement speed)
- z-hop set at 0.3mm and retraction distance and speed at  5mm and 2000mm/min
- reduction of print temp from 220 to 200°C (recommended temp is 190-210°C)
- turning off the heat-bed during print, in case the steppers aren't getting enough current

Start script excerpt:

; current adjustment
M907 X1200 Y1200 Z1100 E1250
; Maximal acceleration
M201 X1000 Y1000 Z200 E1000
; Default acceleration for Print Retract Travel
M204 P1300 R1300 T1300
; Jerk
M205 X8 Y8 Z0.3 E5

All of this helped, but did not solve the problem completely. l I don't think the layers should be shifting at such low movement speeds, which they do. Oozing I can live with, but each one layer shift ruins the print....

Anyone got an idea, what else to check/adjust/replace/hammer?