check gauges ligth comes on when idle

I got a problem with my 02 dodge ram 5.9 check gauges light comes on and OP drops to 0, this happens when the temp gauge is at normal and RPM are at 650 if Rpm are at 700 or temp gauge is a little bit below normal it won't do it, I have replaced the oil pump, the strainer inside the engine and the OP sensor and it still does the same, I want to note that when I drop the oil pan it was like mud inside it looks like they had drop the pan before and when they put it back they overloaded the oil pan gasket with silicone because even the oil strainer was almost clogged with silicone I did clean the oil pan and I could pick up with my two fingers the sludge out of it, any suggestions or ideas of what I should do next, will be very well appreciated, some one told me to drain the oil and add a gallon of diesel into the engine to flush it, should I do that? thank you in advance for you advise.