Mine does something similar, but not as bad. When your oil gets hot, it gets thinner, and pressure at idle gets lower. Three posibilities: bad sender (maybe, not so in my case), worn bearings (most likely, I believe this is my problem), or worn oil pump (not as likely).
I suggest you get a mechanical gage on there ASAP to see if your pressure is really that low. If it is, run some thicker oil (I'm running 15W40 at the moment, IIRC). It will keep the pressure higher, but it will not make the problem go away. It might buy you some time.
If the mechanical gage shows higher pressure, get a new sender. They are easy and cheap.