How's the oil level? Is it down any?
Sounds like the valves or rings are leaking oil and fouling the plug.
Just a thought, but may be good time for a compression check.
However, most 8-cyl. engines don't show much miss if one plug is dead... I hardly noticed any differences in my Lincoln and it's #6 was completly blown!
Good Luck with the 'Hunt'
76 240D (W115) - 550K miles
78 300D (W123) - 200K+ miles