Thanks Larry. I must have corrected the problem as I suspected either the plugs wires might be bad, or the connector on the right side. This morning I removed the plastic side covers on both the left and right sides. I grabbed an old plug that looked good and tested each individual wire one at a time with the engine running. Each plug sparked consistently without any wavoring or skipping. However, I did notice (not sure if it was the wa I was holding the plug) that the two middle wires on both sides didn't seem as strong as the outer wires on each side. Again, probably just a poor observation.The cTonnector C57 is located over the fan on the right side. It is a red 9 pin connector that the 12 volts passes through for the kill switch, then to the coils and ECU. You might want to take a look at that connector checking for corroded or bent pins etc. Also while you have the tupperware off, check C54. It is a white 4 pin connector that all the wires for the coils go through. It is on the right side near the ECU.
Bummer. While your in there check the spark plug boot resistors for corrosion etc.
If you haven't done so already, they are accessed by unscrewing the slotted head that you will see as you look inside the boot, (if you still have OEM wires).
Also make sure that the drain holes in in spark plug wells are open and free of any standing water.
Probably not your issue but quick and easy to check.
Did you pull the plugs right after this happened to see what they looked like? That right there would have told you if it was loaded up with gas from misfiring or carboned up from running rich. It would also tell you what cylinders were an issue.
The spark strength is not effected by the heat range of the plug. That only controls how well the heat is transferred from the ceramic to the metal portion of the plug. If the plug gets too hot, it can cause pre-ignition, and it can damage the internal resistor, if it is a resistor plug, which those are. So you select the heat range so that the plug operates at the correct temperature. Cold weather causes the plug to cool off a little too well, so changing to a plug that doesn't transfer heat as well, puts the plug temp back in spec. Conversely, running at extended high speeds tends to overheat the plug, so selecting one that can move that heat into the head better will keep the business end of the plug from getting too hot.
Put the stock plugs in, run them a while then pull them out and look at them to see how things are running. Who knows why you had a mix of plugs. Might have been what was on hand, or was done for a reason, like you can run a hotter plug on a cyl. with high oil consumption to help keep the plug from fouling as often. Not a fix but more of a band-aid. No idea why you would have two cyl. with colder plugs, at least not on a water cooled engine.