And lubricate it all well and truly for the first few rides, and then about every 400 miles with a good chain lube.
The chain jumping sounds like stretch in the chain, or a stiff chain....eeeeek ! Your sprockets should have flat edges on the part that fits into the chain (upper edge of the teeth) - easy to see on the rear.Look between the links and look for flat s on the end of the teeth. If your teeth have "hooks" it's also a sign of ware - look like shark fins, sweptback
If they look like sharks fins, they're buggered mate !
If you can move the links on your chain when they are seated on the rear sprocket up off the sprocket, the chain is history.......

 

stiff chain got it all cleaned lots of wd40 wire brush seems a lot better now

 

WD 40 on a stretched chain with worn sprockets, worked for me for about
3 days 'til I got fedup listening to the racket it started making and the chit
that ended up all over the rims. WD 40, Oil , tires the stuff and threads most
forums have plenty of discussion about.

If it's OEM at 25k I'd say new sprockets and chain and a decent chain tool
set. (Breaker and Rivet Press in one). Grind the rivet head of the old chain
you are breaking........I've seen a few smart Guys try and press out a chain
rivet with the head on it and break the pressing pin on their nice new $90 tool.

In a hurry takes 30 minutes or a few cold ones some tunes checking out the
piece next door prolly 1 1/2 tops. Tip Clutch piston always relaxes so the
sprocket cover will sit off by 1/2 inch .....line evrthing up and tighten down
evenly and slowly rock the chain back and forward so speedo drive notches
in too.

Not an expert but this nice and easy method worked good for me the last few days

 

the sprockets are in very good shape the bike has'nt done many miles the past few years so the chain was stiff as f**k i will get some new chain lube its about time i started looking after the old girl as she is still in good shape better than myself i must say