On one Quad stack the colour is in register then we speed up the machine and all the colours jump out of register by different amounts upwards on 5mm. Then when the machine is slowed down it comes back into register. Any advice would be welcome.
The first place I would look is for a loose or worn belts/bearings in either the idlers or the drive shafts themselves. Are you running an infeed, is it working properly? If you're running a roll stand are your brake shoes varnished or worn out? If it is a splicer is it holding tension?
Did this problem just start happening or has it been all along? It sounds like floating (A tension problem). I don't know your whole set-up, roll stand and all but that would help to know. Also how tacky is your ink.