fouled plugs

I have a 97 olds bravada V6 with 130,000 miles. My fuel milliage went way down to 6 mpg. The 1,3,and 5 plugs get fouled with black soot when the engine is started. I have replaced the head gaskets, and the normal tune up stuff (ie plugs, wire, cap, rotor) and put in injector cleaner. The engine runs fine at itle and not in gear, but no real power while in gear unless petal is on the floor. What else could cause half of the engine to foul out