Try this the next time it happens... with your foot on the clutch(DOWN)... and your right hand trying to put the shift lever into first gear(but it won't go in!)... with your "other hand" switch off the engine while trying to put the car into 1st gear with your foot down on the clutch.
IF the shift lever goes into 1st gear as the engine spins down... your clutch is bad/worn. Its not releasing. You shouldn't drive too long like this... bc this is very hard on your transmission internals. G.L.