hmm, interesting.

I think there are a couple of different temperature sensors, aren't there? one feeding the temp gauge and another for the EFI? I just had a similar problem on my other vehicle. the EFI's pretty simple on these vehicles, but a bad temp sensor can affect the A/F mixture and possibly cause the problems you're describing. my problem was just a wire that had slipped out of the plastic connector, so it basically wasn't connected to the sensor. I would have driveability issues until it warmed up. it wouldn't even idle on cooler days -- had to start it 3-5 times before it would stay on.

the bucking makes me think bad plug wire, though.

what year is your truck? some late 80's trucks had problems with the temp sensor/dash gauge, can't remember which. I installed an aftermarket gauge in mine, because my gauge would be all over the place and it would freak me out.

do you have any other symptoms of actual overheating? boilover or anything like that?

you might have two problems going on there, but if you're going by the gauge alone, it might be leading you in the wrong direction if the gauge itself is bad. if you're really getting overheating, you might have a failing cooling component causing your temperature climb -- bad thermostat, water pump or clogged radiator.