Just put a new cassette, rear mech & chain on, but when I pedal the mech tends to “flick” backward & forward.
When I have the bike up on the stand all is ok so only occurring under the pressure of me riding, help!!

good chance it’s because everything is new other than the chainring

Had a funny feeling it might be that, budget ain’t gonna stretch to new chainrings either. Gutted.

Replaced chain and cassette yesterday 34 for a 32.
Same thing happened in 1st gear, top jockey wheel was too close to new bigger cassette, so had to adjust derailleur out

A single ring up front? If so you can flip the ring round…

Checked the gap between mech and cassette and it’s ok, running 3×9 so no turning the ring unfortunately.

Stiff link?

As above, worth looking for a stiff link, especially if you haven’t joined it with a quick-link of some kind.

B screw adjustment?

My money is on chainsuck.

B screw set, brand new XT chain with new Powerlink, all running freely, will try new 22 & 32 tooth rings as doesn’t do it on top ring which hardly gets used.
Thanks for all the helpful comments everyone.

New rings fitted today and problem now solved, thank God!