Clutch getting wet on the front of the engine maybe? or the one way bearing on the second gear ( external) clutch pack, it has to be external i dont think its intenral unless your gear box isnt fully sealed? my guess might be the second gear one way bearing,


you just have to use method of elimination

when it happens again, give it alittle gas and confirm the spur gears are spinning if they are not then something is wrong with the engines clutch pack assembly, now look at the second gear shaft, but look at it on the side where the receiver pack is, give it alittle gas when it wont move, the spur gears will start spining, look at the other end of the gearbox and see if that shaft is indeed spinning, if its not, then its the one way bearing on the 2nd gear clutch assembly. Now if those two check out, check the output shafts on the gear box, if those are spinning too, then it could be your set screws that you mentioned, but its probably not that if you checked them already. now you can probably conclude that it is in fact an internal gearbox issue.

good luck hope this helps you find the problem.