Isuzu Repair: 94 Rodeo misfiring at 4000 RPMs, crank shaft position sensor, air fuel ratio

Question
I have a '94 Rodeo ls w/ a 3.2 Liter manual trany.  At 4000 rpms or under any incline the motor misfires and will not exceed 4000 rpms.  I've replaced the fuel filter, plugs, wires, computer, crank shaft position sensor.  I also had the fuel pressure checked and was told it was good.  The Air/Fuel ratio is 18:1.  Timing varies at idle about 20 degrees.  I've also had it on a computer and nothing came up, as the engine light is not on.  If anyone can help please do so.  This is my only mode of transportation.

Answer
Hello Dave. I would start with checking all of your main harness grounds. Isuzu's are horrible for this coming loose. The only thing I can think of is that your evap sensor might have something to do with it. The idling variance might be at fault tho. Have you thought about an IAT sensor chip? If so equppied on your model.