96 Accord interesting problem - Help!

very confused

I replaced the head gasket on my 1996 Accord with a 4 cly Vtec. Now, the engine only runs if my code reader is plugged into the car. If it is pull out, then, the engine dies. I did confirm that it is not a fuel problem.

I recheck all the connectors and they are all plugged in. I also swapped out the computer module from another 96 Accord. The engine still only runs when the code reader is plugged into the car.

Other information: There are no codes set. When the engine is running (with the code reader plugged in) there is some missing at certain RPMs levels. Also, the tachometer runs smoothly at first them starts to jump around a little when I accelerate the engine.

Any ideas? Can someone tell me what should I focus on next or what tests should I do?

Thanks in advanced for any help.