My boyfriend is fairly good at auto repairs --.

My boyfriend is fairly good at auto repairs -- honestly but this has him baffled -- a 1995 chevy truck that he was fixing for someone (catalytic converter, brakes, tune up) was running fine after the repair(s). Now when the car is started and put into gear one can barely move 20 feet and it stalls -- it will start up again but continues to stall -- any suggestions -- isn't there a sensor near or arouind the catalytic converter. After 2 weeks he still thinks he can figure out what's wrong. 18 years ago I had a problem with a Ford Tempo -- did the same thing -- it was the fuel pump AND the fuel filter was clogged -- I can't seem to get him to go that route -- any help would be appreciated