They make a collar you can put on the distributor shaft that will minimize any upward motion the drive gear might have.
After that’s installed, if you still have the timing mark jumping around, it’s likely the loose chain.
They’re often not rock steady on a BB, so how much it jumps around and whether it’s bad or kinda normal is somewhat up for interpretation.
Obviously, you’d like it to be steady.
At some point you’ll run out of things to check……… and be at a point where the cam timing needs to be verified.
But, to have that combo perform as bad as it does……… it’s not a couple of degrees off……. It’s going to have be like a tooth off.