There are two issues here- firstly u have way too much oil going thru yr dripper – 6 drips /min is ample – hence the smoke!!
THe gearbx is definately the problem -and I suspect that a tooth or two has been damaged or broken off – hence the intermittent turnover problem .
In any event taking yr box out is easy – even I can now do this!!!!!!
U can then examine the gears and the bearings.
Incidentally I would think that 40 is too light a gear oil – try 140.