If the cable is reliable with a certain camera then it must be the cable-camera connection that is causing the problem.
Pc connections are unreliable at best. The camera pc port may be worn, leaving the cable loose.

You say your hassy sends the mono's ballistic. That could be a short in the camera connector creating a constant circuit.

In relation to the holga, could the problem be the firing voltage of the radio trigger. The contacts in the shutter would not be the most refined. The voltage in the flash guns may be enough to transfer through the resistance of a bad switch.