1984 Toyota Land Cruiser BJ60 6 Cyl 4.2L
After driving about 60 miles and running perfect the car will begin lurching and hesitating when under any load at all.
load or slight grade. Feathering the gas helps a little. But the speed will reduce to 40 or less. It recovers speed on the downhill side.
I think your fuel pump might be contaminated. Try to replace the fuel filter and observe the changes; if the problem persists the carburetor (if equipped) is dirty.