Express 2005 low pressure oil
 

I bough 2005 Express 4.8 with 141000 miles, when i turned the eng. the oil pressure went to 39psi and gradually dropped to less than 10 psi. I turned the engine off for 10 to 15 second and i turned the engine back on, the oil pressure went up to close to 40 psi the start dropping again to less than 10 psi.
I replace the oil pressure sending unit no success, is doing the same thing.
before i install the new sending unit i replace the oil and the filter, I put 5w10. any advice?

special one