What could be causing my oil needle to be bouncing and then going back to the normal spot it stays at?

I have a 2000 ford explorer, yesterday I noticed the oil needle dropped a about a 1/8 of a inch when I first started it. As I was driving down the road it started to flicker. It didn't go above where the gauge normally sits and didn't drop below where it was at when I first started it. After it warmed up and running for 10 minutes it goes back to where it has always stayed. There are no different sounds and runs great. I even stopped and looked under to see if anything was leaking but didn't see anything.