Oil Pressure Guage

My oil pressure gauge started floating back and forth while on my way into work tonight. It's floating between the 1/4 mark and the 1/2 mark. To be honest I can't recall where the pressure gauge tends to sit normally but I do know that it has never bounced back and forth freely. I don't seem to be leaking any oil and the engine is not knocking. Any ideas?

BTW, I have a '97 GMC Sierra K1500.

Thanks...