plug fouling

[prg machine]

My engine started an intermitent miss and stuttering that came and went and made me think I had valves sticking in the odd numbered (left) bank of the engine . I finaly pulled plugs and number 1 and 3 looked normal with a tan to white insulator and some very faint white ash build up around the ground lug and down inside the barrel. pretty clean for the most part. Number 5 and 7 were fouled with a dark grey to black crusty/carbon build up bridged along the inside of the ground lug and reaching to the insulator. Very hard to remove with a dental pick. Almost welded like. The electrode was not bridged and was still firing for the most part but I could see where this was my problem of misfires on occasion.
I cleaned em up and re-installed and runs great. I need to check the even numbered bank tommorow. The engine has not quite 3,000 miles, still running 30 wt. changed after 500 and once again at another 1000 and due now to change to something different. I was thinking a 10-40 Castrol might be a good choice.
Why just number 5 and 7?
I am running Autolite 26 plugs. I am getting in to a shop for air fuel mixture scope analysis to check the jetting as well. The plug wires are all brand new MSD and are not damaged in any way.
I know this is old wives tale stuff..................but is there some misfire problems between the number 5 and 7 cylinders due to wire routing?


Any advice??

I don't understand why 2 plugs were in this condition unless there was residual break in oil in those 2 cylinders? Engine does not smoke, runs real strong. The misfiring would develop after a period of low speed running or de-acceleration and high vaccuum.