Recently had the engine of my BJ rebuilt (for the 2nd time). Have been testing it for a few weeks. If I am just driving around town there is no noticeable issue. But after about 40 minutes of use, the acceleration gets very erratic... I could be going along a flat road in 4th and the speed starts to decrease even with the pedal to the floor, change to third, still no acceleration, down to 2nd... still the same... then suddenly it will pick up acceleration again. When this happens on a hill, dropping to first gear, the car does not accelerate and has now stalled twice. Then it will drive normally for about 10 minutes or so before the problem starts again. It only seems to happen when the engine is warmed up and has been in use for more than 30 minutes, if I just drive for 10 or 15 minutes there is never a problem. In addition to the full engine rebuild, I have had the fuel tank and lines cleared, new fuel filter, new injectors, air, and oil filter. Any ideas where to start looking? Also, there is a big plume of white smoke when starting the engine, even when the engine is warm. Thanks, Jules