Oil gauge reads normal, then immediately zero?

The oil gauge in my '99 V6 Explorer has been giving me different feedback about my levels after cold starts recently. It did it a couple times last winter, then a few times in the last couple weeks, I'll get in the truck when it's been sitting outside overnight and the oil gauge starts up fine, but drops to zero right away. Then once it warms up, the needle will quickly shoot back up to its normal level, and occasionally twitch a little bit right around that normal level. On a consistent basis I've always confirmed that the levels are where they should be with the dipstick just to make sure the gauge isn't completely berserk. I haven't had the oil changed in many months, so I had already been planning to do that in the next couple weeks. Is that all it is though - dirty oil or something? I have no idea how the reading about the oil level is taken, so is it just gunk on a sensor or something like that or is there a bigger problem? Is there some sort of fluid you can run through the engine after draining the oil to clean out deposits? I'm the original owner s it's always been changed on time and I've never pushed the oil life to the limit, so I'm not worried about extreme sludge or something bad like that (hopefully). Thanks for the help.