fix rear derailleur or replace?

[SteveInBoulder2]

I'm still in the stone-ages of Shimano 10 speed on my main bike. It's 7800 series if that matters.
I've been having less-than satisfactory shifting on the rear derailleur. It is acting like it is hanging up on something. Even if I completely take off all cable tension, the derailleur won't swing all the way to the limit and get the chain on the smallest cog. I can physically move it by hand, but it doesn't seem to smoothly move on it's own under the spring pressure. Shifting is not nearly as crisp and precise as I recall it being in the past. The cable seems to be moving freely and my trouble seems localized to the rear derailleur. I had also replaced the cables as part of annual maintenance just a few hundred miles ago and they appear to move freely.

I've tried to lube the pivots on the arm and there is no change in behavior. My next attempt will probably be to remove it and soak it in degreaser/cleaner. I tried to take the bike in to have someone take care of the derailleur but all the shops around here are either booked >1 month out or won't work on parts as old as a 7800 derailleur.
I can stay in 10-speed land by just ordering a new 105 derailleur.

My question: is it worth the pain/hassle of trying to breathe some new live into the old 7800 derailleur or should I just give up and go with a new 105?