Oil pressure gauge problems

[Hemiterry]

Hey guys ! I’m about to go nuts trying to get an accurate reading on my oil pressure gauge on my 2006 International 5600i with a CM870 ISX. First of all I have a mechanical oil pressure gauge verifying that I have 20psi at idle and 40psi driving. I have replaced the sending unit, electrical connector, and the gauge on the dash. This was all done on a fresh oil change and oil filter. At morning startup the gauge reads 60psi and as the oil warms up it drops down to 10 to 20 when cruising and changes readings erratically when hard on the throttle. The big problem is it sits at about 5 psi at idle when hot which sets off the stop engine and engine warning lights. Once again, my mechanical gauge reads perfect pressure at idle to hard loads. Any help would be greatly appreciated.
Thanks, Terry