Joe's right on the money. If that hydraulic valve is out of whack - linkage or otherwise - that the hand clutch lever connects to, it won't do much at all and will slip. Check the linkage first, then check into rebuilding the valve itself.
We had a 200 having problems like that - turned out to be a worn out bushing in the side of the tranny holding one end of the cross shaft for the hand clutch. We also had problems with our 190XT like that and found out it was a badly worn roll pin holding the control linkage to the shaft (the roll pin broke later in the week, which greatly expedited our finding the problem!).
It may be a more serious (and costly) fix than that, but I'd hate to see you sell it for a $20 repair. Check it out first.