1984 GPz750 - misfiring during warming up

It could be possible that the crimp where the tachometer gets its feed from the black ignition coil signal wire may have gotten loose, causing a poor connection when riding and also explaining the misfire and poor tachometer reading.

bound wrote:

It could be possible that the crimp where the tachometer gets its feed from the black ignition coil signal wire may have gotten loose, causing a poor connection when riding and also explaining the misfire and poor tachometer reading.

According to the wiring diagram for my bike (ZX750A2, non-US or Canadian model), it is the green impulse signal wire that branches off to the tachometer. 

 

If a failing crimp at that junction also temporarily disables the left coil, it doesn't explain why I sometimes have seen lack of signal to the right coil (unlit LED) during some of the stumble/misfiring bouts. 

So, following @Wookie58's guidelines,  I managed to connect a test bulb (for power supply to the coils) and (unfortunately only) one LED across the low tension pins of the right coil (the other, green LED that I intended to use unfortunately got toasted when connected to the battery, since I sourced it from a drawer with old computer parts, it probably was only rated for 5 V).  

 

During my test runs, the test bulb remained lit all the time, indicating that the +12 V supply to the coils was OK. During the usual stumbling/misfiring the LED sometimes remained lit, other times not. And then the engine died completely, also with an unlit LED. After that it took quite a while until I managed to start the engine again. In-between my starting attempts, I held up the ICU and the wires going into it, and flexed them back and forth. This made the LED light up now and then, and even staying lit for a while, indicating continuity at a time when the ignition pickup coils were stationary - which I think of as odd (?) . . . 
Once the engine started again, it kept running without the usual stumbling/misfire. When I got outside my local built-up speed limit zone, I revved it up to 4-6 kRPM. Then the engine ran as it should for a while, until it now and then began running "rough". Loss of power, reluctance to revv, but not complete lack of power, as when it stumbles/misfires at lower (2-3) kRPMs. During this "rough running" the LED remained lit, indicating that (at least) the right coil was firing, also the rev counter continued to show the RPMs the engine was running at (still firing signal to the left coil??). 

The unfortune of having only one working LED makes it difficult to draw conclusions why the engine was running "rough" in-between. My gut feeling says since the left coil did cut out now and then, but I also wonder it it could be due to failure of the advance function of the ICU. At RPMs over 4 kRPM, the ignition pulses are supposed to be at full advance - if they somehow should drop back to 10 deg BTDC, loss of power may be the result?? See - I keep suspecting an intermittent fault of the ICU. Probably in combination with some dodgy connections in the wiring loom.