I agree it's probably not the pump but I would like to know how old it is. Since I always have trouble sealing a pan from under a truck, that would be my last place to check. If this event happens while idling and revving a little, that's good because you can plumb in another gauge in the old gauge location or leave it alone and plumb one in another proper location. If you could see it from drivers seat while driving that would be really great.
Anyway,
I would, in order:
1. Check the oil level of truck
2. Put on another gauge somewhere under hood.
The result of this will lead the way.
3. Change filter if it has one, change oil.
4. Consider that it could be the cold.
5. Go deeper (Jerry)
