Before going too far, have someone SLOWLY work the pedal, while you observe what is going on.
My bet is that the red plastic guide, inside the spring, at the engine-end of the cable has chipped-off, and is thus hanging up in the spring, not allowing full throttle action.
Mine done this about fifteen years ago, actually sticking the throttle at full wide open; then, after carefully digging out the offending broken-off piece, about every third time the throttle would bind up and refuse to go farther.
I carefully re-contoured the open end of the broken plastic sleeve to where it has worked good as new ever since.
As already stated, you can also CAREFULLY break out the entire plastic sleeve and things will work fine.
Also, as old as these trucks are getting, I highly recommend completely taking the cable out, hanging it straight up and down, and flooding it with WD40, or some other moisture-displacing lubricant.
After a good flushing/soaking with the WD40, heat the entire length to pretty hot with a heat-gun, then squirt generous amounts of engine-oil inside the cable-housing.
After mine kept insisting on freezing solid while driving in the cold, I did the above maintenance, and it works better than new.
