Low Oil Pressure

I have a 02 TB that occasionally when the engine is idling the oil pressure drops off to nothing then comes back up to 40. I have tried changing the oil pressure switch, but to no avail it is still occurring. It is strange though cause I can drive for awhile and the oil pressure will be fine, even at idle. Once the oil pressure starts to show low it will continue to drop off when idling until the engine is shut off. Once that is done and it is started back up it will be good for a few minutes again. Anyone have any ideas?

Welcome to AF, the first thing to concern yourself with is whether this is a true reading, a mechanical test gage should be installed in the OP port and monitored for readings.