96 blazer really low oil pressure.. please help

I have a 96 chevy blazer 4.3 that has has oil pressure problems after it warms up. When i first start it, the pressure is about half way on the guage. While it warms up, the pressure guage slowly goes down until my check engine light comes on. If i give it gas then the pressure goes up agian. SO if i am driving it is fine but when i let off of the accelerator it goes down to the red. I have checked the oil and the level is fine. I use a fully synthetic AMSOIL oil but it did it with regular oil before also. Please help me anyone that can. I appreciate it. The engine does not make any noise like knocking or anything. but i am worried its gonna blow up. Any ideas would be appreciated.

Thanks
TJ

Get a manual oil pressure test gauge on it and check real pressure.
Post back real pressure readings.