car stalls above 40 mph

[Quick2000]

Why does my car stall above 40 mph if I hold the clutch in or put it in neutral? When I put the car in neutral, the rpm jumps from 600 ish to 300 ish and keeps going down till it stalls. also, at about 2% throttle, the car is really jumpy and the acceleration is very intermittent.

[randini667]

Is your car an oil burner? Check engine light? I had similar issues when my engine was misfiring from a carbonized/burnt out valve.

[Quick2000]

The other motor was an oil burner, this one has about 8K miles on it...well the block does. I have a check engine light for P0420...catalytic converter is probably clogged from the old motor.