First of all, seafoam will make the plugs worse.....fouls them, but you have the worst possible plugs in your car imaginable. Our 3800'* hate bosch plugs and O2 sensors. Many premature failures, often as soon as 30 days.
Run NGK or AC Delco plugs (NGK makes the Delco'*). And measure the resistance of each plug wire. Should be an obvious progression of resistance from 1-3-5 and 2-4-6 wires.
So even if your plugs were ok, they should be replaced after Seafoam anyway. Your O2 may have also been fouled. Replace it with an AC Delco ONLY.
Part numbers are in Techinfo (do the front O2 only, not the one behind the catalytic converter) and get the sensor from www.gmpartsdirect.com
at about a 40% discount over dealer retail.