bucking / hopping forward intermitently

1990 Toyota 22re deluxe pickup with manual transmission at 237,000 miles.

Truck bucks or lurches forward like a bucking bronco INTERMITENTLY and stops when you take foot off gas. Engine revs and accelerates fine except when this is happening. Took truck to mechanic and it would not act up for him. Bucked on the way home for me. Seems to be doing this more often.

Is this clutch, transmission, differential, u-joints, fuel injectors or what?

Did the mechanic observe how you are shifting or did he do the driving? Does driving conditions matter? Stop and go traffic. Does it do this after being parked a while? or after being driven a while?

Mechanic drove vehicle by himself. Stop and go traffic seems to make it occur more often and more at lower speeds. Has done it at 1st and second gear shortly after beginning to move also, but clutch is fully released.

Also did it once in reverse. Car is usually warmed but it has done it cold.

Sounds like your clutch is grabbing a bit. The problem you describe is common with non-abestos clutch disks. They can take a little getting use to. The next time your truck starts bucking, push in the clutch pedal and then release again. Some clutches require a slow release, some require a quick release.