Sluggo

I'm having some issues with a newly rebuilt Bultaco Alpina clutch(#187). It has a new pushrod, ball bearing, mushroom, basket, hub, plates and steels, springs and throwout bearing. Clutch engages and disengages fine when I adjust it on the bench. But, when I start it and put it in gear I get a terrible squealing and it lurches. It sounds like a bearing trying to seize. I've changed out the throwout bearing and it still happens. Anyone else have this problem?