Janis,
I'm no expert on these engines, but this does sound like an electrical problem. If you haven't already done this, it might be worth checking the battery voltage when the misfiring is occurring. Then turn on enough electrical items to stop the misfire and measure the voltage again.
It's possible that the alternator voltage may be poorly regulated and is rising too high when there is little current being drawn. Excess voltage may upset the ECU. Turning on electrical equipment then causes the voltage to drop into a more normal range and everything is happy.
This may a complete red herring, but it's an easy check to make.
Hope this is of some interest.
Martin
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