2004 Chevy Blazer Oil Pressure Gauge Drops When Vehicle is

My 2004 Blazer is slowly breaking down in various areas, but here is a question I cannot figure out why?

My oil pressure gauge maintains at medium pressure when my Blazer is running at whatever pace, which of course is normail. But when I step on the brakes or come to a temporary stop, my oil pressure gauge drops down to quarter pressure (low pressure), and when I step back on the accelerator, it jumps right back up to medium (normal) pressure. It has been doing this for the last couple days (300 miles). I checked the oil level, it is perfect, and I have clean oil and have always maintained using clean oil.

Any answers on why you believe this is happening would greatly appreciated, and hope I can fix this problem myself.