Is this more than bad spark plugs?

96 Jeep Cherokee 6cyl, p/t 4x4unstable rpm, black exhaust residue, dies when not being idled by someone. In Alaska, a couple months ago, it started acting up, tried to take it to a mechanic, but then it couldn't go over 10mph. With the black exhaust, I tried to change the spark plugs, but since it was -30, I couldn't tell if the wires were plugging in, and I couldn't get to the last one at all. I know that this could be entirely the plugs, but does it sound like anything else as well? Also, it turned out my winterization kit was installed crappy. I've noticed a couple time that the plugs going into the extension cord come out every now and then. If you are unfamiliar with Alaska, we have to install heater elements on the engine block, oil pan, and battery; so that's what I'm talking about there.