I have a 2004 Tahoe with a 5.3 flex fuel engine and I've started to have low oil pressure issues at idle when hot only. When I start the truck cold the oil pressure is at 40psi and will go higher whenever I touch the accelerator. But as the temperature starts to rise the pressure will slowly bleed off. At 210 degrees the oil pressure is at 20psi and will go up to 40psi at about 1200 rpms. I've already replaced the oil pump pickup tube o-ring thinking that was it and it didn't solve my problems. Anyone have any ideas?