Switches can be a source of big headaches I have found. You might try bypassing the switch and plugging the batteries directly inot their respective items, both reciever and ignition. After do this run the test again and see if everything is clean then plug them back inot the switch, one at a time, till the problem shows up again.

The single biggest culprit though is extensions, namely the plugs, as they can make intermittent contact while subjected to varying amounts of vibration. The weeding out process is the same as that described above, unplug everything and then test each thing one at a time till you find the offender.

I don't recall if you said you were using PCM or FM in this model. Regardless you need to be using an FM during the testing phase as the PCM will do it's job and show no signs of trouble, making it almost impossible to accurately determine the cause of the problem.