Truck Is Cutting Out.

[Giantmover]

Hi, I have a 91 Silverado with a 5.7. Motor has never been touched. My problem is, when I am driving down the road and the motor will drop rpm's and start to diesel. It will then go back to normal rpm's then drop again diesel then eventually shut off. I then put it in natural start it and it fires right up. When I am idling in park it just shuts down no dieseling just cuts off . I turn the key and it fires right up run for a bit and shuts down again. I have replaced the ignition control module and the pickup and it still does it. I have put better gas in it , same thing. I have watched the injectors while it is idling to see if they stop spaying to see if it stops getting fuel, they dont stop. I brought it to a buddys garage and put on a computer the only code showing is the egr code. Is there anyone out there that could possibly shed some light on my situation. If so I will greatly appreciate it. Thank you for your time.

[Wingnut]

On a '91, I'd be replacing all the 02 sensors, knock sensors, etc...I know that the knock sensor on my '92 handicapped the hell out of it when it went south.

[skills4lou]

When was the fuel filter last replaced?