provided the engine is still revving up ok and not just stuck idling, best advice i can give is to strip the entire drivetrain down, clean it all up, and see if anything materialises while u are at it. check that ALL the bearings are ok in the diffs, and clutch etc, and as u reassemble as per factory instructions, make sure none of the parts are binding, or that anything feels stiff.

it sounds from ur description like something is getting hot and binding, then it cools and frees off, like rolland mentioned above.

check that 2nd gear clutch pin i mentioned too.

let us know how u get on mate, whatever u do, good luck