Random cylinder firing

2004 Nissan Frontier XE 3.3 auto 97k miles. Truck idles around 9k then drops to ~5.9k shaking/bucking for ~10-15 seconds then goes back to good idle. Appears to be consistent pattern. When drops down to ~5.9k significant loss of power until stabilizes back to ~9k.
have replaced distributor, checked plug wires, for vacuum leaks, fuel injectors (repaired 2 that were spitting fuel).
have heard that Nissans (esp. Frontiers) have reputation for running into numerous faulty distributors (replacement distributors). Have even heard nissan factory mechanics say they have changed 5-6 OEM distributors before getting a good one. Gas tank, fuel filter and sending unit all replaced previously (and vehicle operated fine after). Crankshaft position sensor replaced. Mass Air Flow Sensor cleaned.
flashing error code (knock sensor and random cylinder firings)
Appreciate thoughts on this.