I hear you describing an engine that is surging--pulsating in speed with the throttle being steady.
The clutch is less prone to slip in lower gears.If you can spin the wheels in lower gears there should be no problem attributed to slippage--in lower gears.
Clutch "chatter" or "judder" occurs as it is being slowly engaged but goes away after engagement.
Clutch "drag" occurs when trying to dis engage and the gears clash or the machine trys to keep pulling.
When it jerks , cold with the clutch (pedal) out, probably the engine is stumbling due to incomplete warmup.
Have you mastered the art of modulating/varying the choke to drive away smoothly with the engine cold?-- My guess is; not very well.
Don't feel bad, most people raised in a time when engines had automatic chokes or fuel injection were taxed to learn the art.Why, even when all had manual chokes there were many who never learned what their engine was telling them it wanted to run smoothly under varying speed and load.They just thought choke on to start then off after that.
