help please

My Yukon has about 293000 miles on it. I've recently noticed the pressure sitting right over 40-45. It then dropped down to 0 and turned off. I Couldn't get it to start. Went ahead and changed o-ring in pickup tube and oil pump as well as water pump since I already had it off. Now at start it goes to 50 then drops once it's warmed up (about 10-15 minutes) to 15 psi. I put 10-40 oil hoping it would help but, nothing changed. What else could cause this to happen.