If the coil packs and leads are the same part number on each bank, swap them all over.

Simple to do, and if the fault moves over to the other bank you know where the problem lies. If it doesn't, then its not the coils or leads. Then try the spark plugs.

Also, seeing as how you have a problem with crap accumulating under the oil filler cap, it may be prudent to consider the possibility that you have a cooling problem on one side of the engine.

I would think carefully before 'testing' her again.