1996 dodge grand caravan

I own a 1996 dodge grand caravan se, 3.3lt with 246,000 mi. It will run fine for about 40 miles. then after 40 miles or so it begins jerking and missing. we have replaced the computer, fuel pump, fuel filter, sensor, and catalytic converter and, oh yeah we put fuel injector cleaner in it too. The timing set. Does anyone have any suggestions,ideas for why this may be happening?

Spark plugs? Also a fuel pressure test when it is jerking/hot. Maybe you have a bad fuel pump. Any codes in the computer? Also why did you change the computer to begin with?