Unfortunately a possible scenario is that your oil pressure is actually dropping below 5 or 6 psi, and since the sender is an "on/off" unit, what you're seeing is as "bouncing" is actually poor oil pressure warm and at idle. The gauge on our cars is just an idiot-light, which would be flashing if it were one. In my experience this is pretty common on a worn engine (bearings well beyond their specs), much less likely due to a worn oil pump.
The best way to rule that out is to thread in a real oil pressure gauge and see what's happening. Otherwise you could try replacing the sending unit. If that doesn't work, using a thicker oil can help bring your pressure back up a bit. But the end result is a rebuild might not be too far away.
What's your general engine condition / compression / oil consumption / mileage?