Car studdering again

I pulled a plug & noticed it was a little "black" on the tip. I should note
that the plugs in there are NGK Iridium spark plugs. I don't know if this is
normal or not. On acceleration, the car "studders" so to speak. Last 3-4
times it has done this.. it has been ignition related. I've replaced the
spark plugs & it went away for about a day... only to discover that the plug
wires were bad after testing them. So I replaced the plug wires... problem
went away for a few weeks, IIRC. Came back... replaced spark plugs with NGK
Iridium spark plugs & has been a true champ for months. But... the dreaded
STUDDER is back. Car is pinging on 89 at full throttle too so I have been
using 91. Latest things done was pouring Seafoam cleaner into gas tank.
Also.. replaced the clutch... uhh... can't think of anything else. What
could be causing my plugs to f*ck up each time like this? this is gettin
expensive to keep replacing these plugs & I'm tired of busting my fscking
nuckles!! Thanks in advance. Btw, flows sound great :x.

-Mike

--
A happy kid behind the wheel of a 98 Mustang GT
Cold air intake
FRPP 3.73 gears
Steeda Tri-Ax Shifter
Flowmaster 40 Series mufflers (self-installed woohoo)
Hi-speed fan switch
255/60R-15 rear tires
Subframe connectors