Since changing the clutch on my A1P the bike is jumping in and out of 1st gear!

It only happens on over half throttle at around 7,000rpm and 10,000rpm but it's so heavy that it tries to bring the front wheel off the ground!

I've been told it's definitely the gearbox as it only happens in first gear but it never used to happen before the clucth was changed. It feels like the clutch is being blipped then catches hold and tries to lift!

HELP!

Any advice would be appreciated as I'm tempted to sell it and cut my losses but am gutted at the thought of it!