Oil Pressure Puzzler

[palallin]

Background:
2001 Tahoe, 4WD. 5.3 LS. 245,xxx miles.

Twice since the weather has gotten cold, I've had a scare. I start the vehicle to warm up, leave it for a moment, and come back to it to see the Oil Pressure Warning light on and the gauge reading near (but not at) zero. I shut it down. I check the oil. In both cases, it was less than completely full but oil was on the stick (the first time, just over a quart low; the second time, just under a quart low--oil consumption has been increasing). I topped off the oil.

At restart, the gauge comes up. It hits the 40 mark at idle as it warms up. The warmer it gets, the higher the pressure reads--which is the opposite of all other vehicles I've ever owned, which lose a bit of pressure as the engine warms up. At no time when the light was on did the engine make any noise, run at all rough, or indicate high temps. There are no after effects of any kind other than the wonky pressure readings.

So my puzzle is this: am I dealing with an oil pump problem, some kind of clogged passage, or a screwed up gauge? Does the gauge on these actually have pressure to it, or is it electronic?

Any input would be welcome and appreciated.