I have seen a few MTX-75's with leaks around the case gasket. Although slow there is still fluid leaving the case and over time, without keeping a check on the fluid level, it will eventually run low enough that the gears are not able to get the bath they need of fluid.
I guess it could be also possible that one of the axle seals went bad, allowing the fluid to quickly come out of the tranny. That would explain the smoking that you mentioned, since the fluid exiting the tranny would come out on the axle which would throw it all over the place under the hood. At the sametime, once you came to a stop it would just pour out since the shaft would settle at the base of the seal and not float in it.
