Hopefully theres something simple im missing here…

Im used to sorted out my mtb with 1x drivetrain, never had a problem.

Bought a second hand road bike which worked fine but nnoticed the chain and cassette were very worn. Replaced both of these, I can now get the rear end to index up properly on either / or of the front chain rings. If i get it running silky on the 52 it jumps and messes about once dropped to the 36.

Any suggestions?

Does it index up and down cleanly? If not, suspect cables.

Sticking pivot in the rear mech-works under tension when in the outer ring, doesn’t want to play when you drop it into the inner.
Might not be it, but something I’ve seen a few times.

Another possibility if you have internal routing is one inner wrapped around the other, and tensioning/de-tensioning the rear cable when you shift at the front. It’s rare, but it does sometimes happen.

It does perfectly on either ring, just cant get it to do the same on both chainrings on the same setup. It is all internally routed so i guess that could be the issue. Thanks

Replace the chainrings as well.

Bent rear mech hanger ?
I had grief with mine the other week, couldn’t get it right myself despite all new chainset and cables, gave up and took it to my lbs.
They said the hanger was way out and adjusted it, it’s fine now.
Then I remembered the bike falling over heavily on the mech side about the time it started to play up.