My first thoughts are that the gearbox has been rebuilt incorrectly at some point.
Maybe a thrust washer or two in the wrong place, or missing even? This could possibly cause some binding of the gear cluster when the engine is running. The primary drive gears are helical cut, maybe something is amiss there, causing some side thrust under load?
If it was me, I'd whip to box out and check that it's been assembled correctly.
Last time I checked one I used the parts manual to check the position of the various components.
You need to get the thrust washers the right way round too. The rounded edges from where they've been punched out need to face the gear. The sharp edge should face the circlip.
One thing worth checking is the seating of the spring for the selector drum positioning arm. It has a roller that sits in the indents to hold the drum in position.
When refitting the cassette, you need to make sure you push the rear end of the spring into its seat on the crankcase. If you don`t do this it sits between the mating faces of the cassette mounting plate and the crankcase. This distorts the plate, in turn, loading and binding the shafts and making the gearbox very stiff and hard to operate.
I`d check this first.
Sorry for the essay!