Jeep spark collapse. Is it PCM or something else?

A 1996 Jeep Cherokee 6 cylinder. The condition is RPM decreases, which is caused by observable spark decay which coincides with RPM drop, then drops below minimum RPM, approaches stalll, then recovers. I checked CKS, replaced CPS, replaced coil. Tomorrow I will test TPS. What am I missing? The Manual says to take to dealer for PCM check. Any alternatives?