Cooling fan issues

I have a 2005 379X Pete C-15. The cooling fan comes on and off randomly, the temp gauge reads good temp. it will come on and run 2,3,4,5, min or stay on longer or all day. I can be idle it will come on while its 0 deg out side. when it comes on it will cool the engine to below 150 deg going down the road.When I shut the truck off you can hear the fan solinoid release air. I done a diag. from the dash and got a coolant temp sensor fault. I had it into Peterbilt Mississauga, they said it was the ECM and had moisture in it. , I had it removed and tested and it seemed fine other than coolant temp sensor faults. it was reinstalled and the coolant temp sensor replaced. it seemed ok when I picked the truck up but now its doing the same thing. When its not on , I can run it off and on from the dash fine. I have checked the A\C switches are off and temp controls are on heat. coolant is full no other warnings or faults.
Thank you,