Boxerworks.com

Good evening,

When I take my 79 r100t out for the first hour or so, neutral, engaging the clutch into first and pulling away is fine. Then after an hour or so, when half releasing the clutch, pulling away and easing the clutch out sees a jump - not on mild acceleration but pulling at a moderate to fast pace like roundabouts. It's as if the bike engages and then re-engages first gear. All other gears ars perfect and it appears to only happen once the bike is warmed up. Help greatly appreciated

If it was a driveshaft/clutch/spline issue, it would also do it in second gear when similar torque is applied.
You have an internal transmission issue. (99.999% probability) I've fixed this problem more than once on customer bikes.
Coincidentally they were all very high mileage bikes.
It’s impossible to estimate the damage and/or cost without an autopsy.
It depends on the transmission’s overall condition and what exactly needs replacing.
Most likely you'll only need a gasket, a few gears and bearings, and shims, and seals, and some springs.

gspd has a history of knowing what he's writing about.

However, first thing I'd do is check
1. clutch adjustment
2. gearshift isn't loose. Simple and easy to do...
then
3. as per gspd

Roger, If you’re not ready for a transmission rebuild for whatever reason, change the transmission oil (when hot) and see what comes out. If no chunks fall out and there’s no crap on the magnetic drain plug you can probably get many thousands of miles of riding out of it if you ‘adjust’ your riding style to baby first gear so it doesn’t jump out. Not something I really condone, but I know somebody who’s been doing that for over a decade without any bad consequences (so far). He’s old and poor and only does about five thousand miles a year on his neglected rat bike airhead that also has a million other things that could use some TLC (err, cash), but the damn thing just keeps on running.