Do you have a good feeling on the pedal? When you engage the clutch, does it engage smoothly at the friction point? Does the clutch disengage completely? If it just can't hold when torque is applied on it, it is possible that the pressure plate doesn't give enough pressure on the disc, or the mating surfaces lost friction over time. The disc could be glazed. Sure it is funny that the clutch started acting like this after a long period of not using it, maybe it was just about to slip.
I broke the clutch on my ST185, once, and figured out that there was no other way to do it, than taking out the whole power train. Anyway, if you have to change the clutch or get there, it's not a bigger job to change the crank seal. It take 2 more minutes... I highly recommend that you change the seal, even if it's not leaking, when doing the clutch, being preventive. You don't want to crack the engine from the transmission twice in a short period of time...