You could do with a swap with another owner, to see if the fault moves with it.
I guess the sender is the easiest.
I expect not many would wan't to remove their's as per " If it ain't broke, don't fix it" principle.
If you knew what pulses the sender sent, ie at what voltage and frequency, it could be possible to simulate the signal for various revs, this is how I calibrated and modified a standard car type tachometer for my outboard.
If the signal is analogue ie voltage/ frequency, then it would be possible to see the signal on an oscilloscope.
The fact it is intermittant, does imply a faulty connection, but this could be at either end.
Rustic