Derailleur, chain or something else?

Started noticing rear derailleur skipping or like half shifting up and then dropping down on some short local rides after I took an extended break since last fall. Had been averaging 100-200 miles per week for about a year or so and had experienced nothing like this. I did just transfer this derailleur to an old frame but it was happening before the move too. I was hoping a fresh set up would improve but alas the situation has only gotten worse. I’m no expert so there could be something I’m overlooking but any ideas would be most appreciated.