Low oil pressure

I have a 94 lesabre, and my problem is when I first start driving the oil pressure seems to be where it always was when i first bought it..Now after driving for 10 minutes the oil pressure gauge starts to go down. When I stop at a light the gauge drops to the line just before it hits the red zone. When I accelerate the gauge goes up a little. Also is there a site that has a diagram/pictues of where certain parts are like the oil pressure sending unit, the oil pressure relief valve.

Thanks