Suggest you take a resistants reading on each plug to ground. Not sure what the reading should be but they should all be about the same, maybe the manual will specify the reading. Then measure the voltage at each plug with the switch on, should be steady.
Check all electrical connections, plugs draw a lot of current. If fed through a solonoid consider replacing that if nothing else is found. Fuel maybe? if you can see the filter(s) bowl it should be full, no air in it. Check allconnections, air leaks are hard to find. Let us know what youi find.