Before cars had disk brakes the drum brakes would overheat and in effect lose all the braking power. I had this happen once to me when a clutch overheated in heavy stop and go Jersey Shore traffic jam on the GSP. I think your slipping clutch overheats and stops working at all until it cools down.
Remedy is a new clutch. There could be some weak springs in the pressure plate so I’d get a new pressure plate in the job.