engine ignition seems to shut off and the car hesitates and bucks?

I have an intermittent problem that does not seem associated to temperature, environment, and is most visible at highway speeds but also happens at idle occasionally. The engine ignition seems to die and the car hesitates and bucks. This can be seen in the tachometer, which briefly starts falling to zero. Also, when the problem occurs, the ammeter drops down and slowly returns. After the problem occurs, the MIL will illuminate the next time the car is started but can be turned off by killing the engine and restarting it. With an Actron scan tool, I found 2 codes in the system. They are 321 and 361. The scanner said that 321 is "18x interrupts/ignition" and 361 is "EST not toggling". Studying the wiring diagram shows that an EST signal goes into the ignition module and that the EST signal uses the crankshaft sensor and camshaft sensor as reference signals. I replaced the crankshaft sensor and then the ignition module but the problem is still there. Any help would be appreciated