Once in a while tranny will not engage engine revs then next time it will go replaced primary clutch slipper ok could it be the one way bearing in tranny?
Try that, or the primary clutch inside the tranny....check teeth on gears inside etc etc..
Jato 3.3 Pede 2.5R Rusty 3.3
Mine was usually 1st gear when that happened, went steel. No more problems.