I upgraded my rear Derailleur and shifters from a Sram X4 to a Sram X7. I've upgraded my Cassette and Chain to Sram 951. Now when I Shift into 3rd on my my front sproket my chain skips and falls off. My high gears seam solid. It seems like my chain is to long on the low gears and to tight on the high gears. The X4 seemed more solid and smoother.
When you installed the new chain, did you remove any links from it?
Look, whatever happens, don't fight the mountain.
Tests for correct chain length first.
It should be long enough to loop the large/large combo, plus 2 links (1") minimum, without straining the RD cage. If you have suspension, flex it through its full range to make sure the chain is still long enough at the tightest part of the arc.
It should be short enough to loop the small/small combo with the RD taking up all the slack.
If the same length can't pass both tests, you need a longer cage RD.
If it's marginally close, check the "B" screw adjustment and see if bringing the RD back or forwards solves the problem.
Lastly, a matter of personal preference, if a range of chain lengths can pass both tests, I prefer to go with a longer chain. This gives me the possibility of doing an emergency field repair by cutting out 2 links. Many others prefer the shortest chain possible, that's your call.
The key to solving any problem is to understand and address the underlying cause.
Also be sure your rear der cable and housing is not worn out and hanging up. That's a hidden problem for rear gear skipping and alignment problems.
Cable housings, especially sections where they bend and the cable wears the inner plastic faster, should be replaced at least once a year for those who ride a lot.
My cables tend to wear out at the shifter end about every year too, so I replace both cables and housings every year as regular maintenance.
I took my bike in for some adjustments and it was to wet out today to give it a good test run. The chain length seems to be correct. I may have to change my fron sprocket The current one needst to make a full rotation to catch. my smaller sprokets only need to make 1/3 a rotation and they catch fast. I plan to test it tomorrow after work.