Silverado 98 PCM without cylinders 3 and 4 signal

Greetings to all, I am a technician in repair of automobile electronic control modules and I am starting my own business in this field. My question; I have in my workshop a Silverado year 98 automatic that presents a very curious fault; The PCM stops sending activation signal for injectors 3 and 4 and accelerates the pulse for cylinder 6 injector, but this failure only occurs in the morning, I have already replaced the respective transistors, even the micro controller of them and the fault remains the same. In addition, I already tried another PCM and follow the same fault and the most curious thing in the case is that two trucks are already there with the same failure this week. Could someone please, with due respect, orient to solve this fault? I will be very grateful.

If with two computers the fault is the same then this drawer that the fault is not the computer, it could bother any sensor either temperature or position and uncontroll the parameters of the PCM

Thank you for your suggestions and interest in helping me friend Kikemec. Indeed, it was the cam sensor (inside the distributor), which was causing me that annoying failure. I hope this data helps for everyone who has this same problem. Greetings.