I’ve recently replaced an old worn out 8spd setup with a 9spd using a collection of parts I had, along with some new ones:

New:
PC-971 chain
SLX Rear mech
Deore chainrings

Used:
SLX shifters
XT front mech
Deore 11-32 cassette

I’ve indexed them as best I can, but the shifting is not amazing, possibly because the gear cables have to be so long, but thats not the main issue. For most of the gears, as soon as any power is put down the chain will skip on the cassette, as if it was trying to change gear. It does this no matter how I adjust the indexing or try and trim the mech. I’ve also tried adjusting the B-screw but this seems to make very little difference. I might put this down to it being a new chain on a used cassette, but the cassette probably only has <100 miles on it and looks basically new. Is there anything I can try short of buying a new cassette?

Edited to add new chainrings.

Is it possibly the rings on the crank rather than the cassette? It can feel like jumping on then cassette

Ah I knew I’d missed something. The chainrings are also brand new deore rings. As far as I can tell its the cassette where the skipping is happening, although it’s more difficult to tell when I’m way up front. It is also equally a problem no matter which front chainring is used.

Freehub on wheel slipping?

You could be on to something there. This is on a Deore hub that is fairly old and has only been used on a standard bike (so half the power).

Bent gear hanger maybe ?

New SLX rear mech (i.e. 10 speed?) with an old shifter may be the problem? Didn’t shimano change the amount of cable pull needed on rear shifters when they went from 9 to 10 speed, so you can’t use a 10 speed rear mech with a 9 speed shifter and vice-versa? I had similar problems on a cannondale tandem once caused by that. Try getting hold of an older 9 speed rear mech to solve the problem?

Otherwise bent gear hanger is your next most likely candidate. Or maybe play in the rear wheel bearings or freebuh body.

Id try the a new cassette.

It’s a 9speed mech, and the hanger appears to be straight.

Old cassette – new chain. Thatll slip right enough