Does the 6 have carbs?
If yes, it could be running too rich, or it could be running too lean, the jets could be the wrong size, the mixture adjustment screws could be wound out too far, the throttle valves could be out of alignment, the throttle shaft springs could be shot, etc, etc...
Is this a new issue or has it been going on for a while?
If you're mechanically minded you can tune them and even rebuild them yourself, it just takes the internet, patience, a lot of adjustment, and a syncrometer.
Whether they'll stay tuned is another story