I had a recent issue like this as well. It turned out to be a cable housing issue just as Bill talked about. I removed the RD and soaked it in solvent and dried and lubed it. But still getting stuck. Looking at the cable housing I found a pinched end underneath the cable housing caps on two of the cuts, and the plastic guide under the BB was grooved like I've never seen. But luckily it was a fairly standard part I have in stock and easily unscrewed it and replaced it. After fixing those issues and replacing the fat, stock Shimano braided cable with a cheap, thinner $0.50 cable, the shifting works great now. I think the issue was also, non-shimano housing with shimano OEM thick cable.