Wow, I would have been wiling to bet quite a bit on crankshaft balance. All the classic symptoms were there.



The coil - put the ohmmeter across the two input terminals, you say it is. Next put one ohmmeter in the plug wire hole and the other to a bare spot on the coil If you have continuity it's okay, too. I have never heard of coils cutting out at certain rpm levels.

You have checked continuity of all plug wires and they are all able to conduct electricity.

You have checked continuity of the pickup coil, and it's good.

Have you checked the spark plugs? Occasionally one is bad right out of the box.

You have wired the heavy red wire to battery positive post, connected ignition box to cab. So what connects the cab to the battery, ground-wise? Running a black wire all the way to battery negative is a good way to find out about the grounds. You have mounted the box in the cab. What kind of wire is the signal coming through from the distributor pickup coil? You have a moderately long pickup coil lead and it could be giving you troubles. For example, under certain conditions the pickup wire can act as an antenna and the voltage it gathers can fool the sensor circuit in the ignition box. Under other conditions the pickup coil lead may make a capacitor with the floor sheetmetal. This could be enough to send the system into oscillation.

I'd suggest a twisted shielded pair from distributor to ignition box.

There is a chance that the box has something wrong internally. I don't think there are any tests you can run unless you have an oscilloscope. You'd also need a signal source to drive the ignition box.

As this seems to be an rpm-related thing, have you tried setting it at different speeds and seeing what happened?

Good Luck!
R.