Okay, stupid question to try and defend the sliver of hope I'm standing on. Did you possibly switch wires? Remove them from the coil towers and when you reinstalled them possibly switched #2 and #4?

Nope. When I replaced the first cracked plug, I pulled the one wire, replaced the plug, and replaced the wire. I didn't swap any around.

However, I will take a look at #4 while I'm in there and see if it has any marking that might indicate that this effect is happening. If needed, I have spare wires for this car and I can swap fresh ones in.

oh well, i guess i lose

I'm wondering if it isn't a heat issue. I was more expecting that it would be #6, but #2 and #4 could still be affected. Are the metal covers for the spark plug boot still there?

Yep, boots are there, however I tend to think heat is at least a contributing factor too. Both times this has started while driving up steep hills on hot days and the motor was running warmer than normal. Not overheating yet, but the gauge was well over 200.

I've never used Autolite, so I can't speak first hand as to their quality. I also have no reason to believe they aren't a quality plug. But, you may want to try the NGKs as a test, if you haven't already. Your unique set of circumstances may be causing you a problem that no one else will experience. You may want to find a way to keep your engine cooler under extreme conditions. Aftermarket PCM, drilled 180 thermostat if you haven't already, or a switch to turn your fans on manually.

I have a 180 in the garage ready to go in for the summer and a bottle of Prestone Radiator Flush since I believe my radiator is a bit gummed up. When I refill it, it will be with Water Wetter to improve the heat transfer of the coolant. The fans work, they just aren't sufficient to keep it as cool as I would like in harsh conditions. Its frustrating. My Regal will stay at 180* climbing hills, sitting in traffic, AC on, AC off, it doesn't matter. The Bonneville, with what should be an easier to cool motor, can't hack it. It could be that the Autolites, while otherwise good plugs, can't handle excessive heat. Which would explain why my Regal uses them just fine and the Bonneville can't. I will be trying NGK when I change them this weekend. If nothing else, I should see better gas mileage with fresh plugs.

A third one cracked. I had replaced the last cracked one with another Autolite I had around just until I could get my hands on some NGKs to replace the whole set with. Last week, the miss started again, this time on the #3 cylinder. I got my NGKs earlier this week so I did the complete swap this weekend. Sure enough, #3 was cracked, not as bad as the others, but two very small cracks in the insulator. On top of that, the Autozone plug wires I had been using completely fell apart when I removed them from the #1 and #3 cylinders. Again, due to previous failures of these wires, I had a spare set around, so I replaced them all as well. At this point, I would not recommend Autolites to anyone running an L36 powered Bonneville. They seem to work fine on the L67 in my Regal, but something about the L36 just hates those plugs. We will see if the NGKs fare any better.