I just bought an 86 toyota pickup off of my buddy for a good gas mileage truck to get me back to forth to work. He told me it needed new plugs, but when I started driving it, it seemed fine. It had plenty of throttle response, and would sit at idle all day long.................until I was going to my girlfriends house this weekend. She lives about 40 miles from me which requires alot of highway driving. As I was heading down 95 it bucked a couple of times when I got on the gas. The further down the highway I progressed the worst it got. It got real bad up any sort of hill. It would buck like a bastard. I changed the plugs today (the old ones looked fine), and took it for a ride with no problems.....................until I was again on the same highway heading home from my girlfriends when it started. I even sat in about twenty minutes of bumper to bumper traffic with no problems. It seems to only do this when it's been running for an extended period of time. As soon as I push the clutch in while its bucking it idles fine. Once the bucking starts it gets really bad when it's under a load. I'm thinking it's the cat. I had a new muffler and tailpipe installed the other day, but the cat. is original. Any info would help!!!!

if it starts bucking/sputtering under loads or going up hills, it sounds like it could be a clogged fuel filter, fuel pump going out, or your carb has its float level set too low.

You also might take a look at your spark plug wires. They could be just out of spec enough that the prolonged heat from long drives makes them perform badly. I know my carbie bucked like someone was dancing on the brakes when the original wires finally died after 17 years.

OEM, NGK, or Denso are the best choices if you do determine them to be the problem.

I'd have to agree with the Fuel supply being a problem. Had a similar problem years ago, was a old fuel line suctioning closed during freeway driving or under load. Car would idle all day long and in town driving was no problem, but as soon as I got on the gas it would sputter and buck like a bronco.

I get my gas from the cardlock in town cuz its cheap but I get symptoms like yours if I use their premium gas. Evidently no one else does and it sits and gets water in it.
Edy from "Behind the Redwood Curtain"

Not that this is the exact same as your problem, but my '83 would buck when Iunder a load when cold and then also at low engine speeds like in parking lots. I adjusted my valves, and the problem went away. Very cheap and easy thing to do.