fix rear derailleur or replace?
 

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?

If the derailleur moves freely elsewhere in it travel, and just gets hung up in the last millimeter of travel outward to the small sprocket, then one simple thing to try is inserting a washer on the mounting bolt (between derailleur and frame). This will result in more spring force through out the entire derailleur swing (including toward the small sprocket).

If you orient the cable clamp bolt washer 90° from its proper orientation, it will bump into one of the pivot knuckles in the smallest gear and prevent good shifting. Loosen the cable anchor bolt, spin the washer 90°, retighten the bolt and your r.mech should move a bit further to the right.

EDIT: pics added. Looks like it could be either 90° or 180° off. The tab should be at the 9o'clock position when looking at the derailleur from the "outside" of the bike.

(I have an old unhealed mental scar from this puzzle!!!)

sell the rest and buy a new group. better shifting and availability

Lube the pivots.

Look585 was spot on...

I had initially ordered a 105 deraileur to replace the 7800 one. When I started taking apart the existing stuff, I saw where the longer bent cable retainer washer side was rubbing against the pivot arm. I rotated it and am now once again happy with the shifting on my trusty old steed.

Thanks Look585!