Rapid rise rear mech.

I fitted the rear mech and no matter what i did i couldn't get it to index properly, it would change from say 1st to 2nd then jump to 4th.
I gave up out of sheer frustration, but having said that i did see how quickly it changed down gear and i can see the potential.
Anybody any ideas on fixing the indexing problem.

They index in exactly the same way as conventional mechs.
So fixes would be new cables, straightened hangers, making sure the cable is on the right side of the pinch bolt and so on.....

I've tried all that, i went through everything to make sure it was all aligned and used a new cable, i have noticed since taking it off, that the wheel that assists the cable is chewed up a bit and not running smoothly, i'll sort that out and try again.

If the wheels knackered, it may be curtains for the mech, i've had them jam in the past, need careful cleaning and keeping free running.
I suspect that is what your issue will be.