Under (and Over) Pressure

On the way home from BASFA tonight, the oil-pressure gauge on my van started fluctuating wildly, including pinning at the top end for a while. I pulled off the freeway to a shopping center parking lot and checked the oil. It didn't seem low -- possibly at the low end of normal -- but it's hard to check the level on my van's engine when it's been running. I started it up again and the pressure reading stabilized near the center, so I got back on the freeway for the remaining six or seven miles. As I neared my exit, the needle started wobbling again.

Tomorrow morning, I'll check the level after it's had the opportunity to settle down. If it's not so low that further operation would be potentially damaging, I'll take it over to Jiffy Lube for a top-off. But I'm concerned about why it should have gotten this low (if that's the problem) in the first place.