I do fine with
holding a heading, but I find it very difficult to maintain an
altitude.


That's not really what you can practice with MSFS. It's just not close
enough to the real thing.

What you CAN practice, is procedures. The whole timing, cockpit
organisation and everything. Figuring out hold entries, setting up
stuff, how much time is there from IAF to FAF to touchdown, those
things. You need to really sit down with the kneeboard and approach
charts. I wouldn't/didn't bother with radio work, since it is so
unrealistic.

If you do this with approached you actually use during training, that
helps quite a bit.

If you want the sim to be smoother, try dialing back on the graphics
settings. You don't need dynamic scenery, you don't need cloud detail
or scenery detail. That helps a lot.


--
Thomas Borchert (EDDH)