Sounds like the plugs could certainly be part of the problem. My stock plugs at under 50K had the ground electrodes scalloped like a spoon. Autolite double platinums, APP764 IIRC, should be your next plug.
You still seem to be running lean, so please also check the brake booster vac line, the PCV system, your air filter, the vac tube under the UIM (just behind the throttle body), and clean the MAF sensor.
Check your plugs after a week of running & if the above doesn't reveal positive results, check your other vac lines & the rigid lines on top of the UIM & thier elbows. They crack sometimes. Run for a week.
Still no joy? tighten the UIM bolts to the top of the specified range, BUT PLEASE NOTE THE RANGE IS IN INCH-POUNDS. If you over tighten them you risk stripping the aluminum head, so be firm but not forceful.
