[cdru]

My first guess would be a failing oil pump or related component. At idle speeds, the oil is at minimal pressure. By revving the engines slightly, it pumps more oil, increasing the pressure.

It could also be a failing pressure sensor, plugged passage, etc...

My 97 did a similar type of thing as I was driving it home from the place where I bought it. I limped it home and the first thing that I did was did a complete oil flush. The car had been neglected for some time from the previous owner and the new oil has seemed to breath new life into it. The oil pressure light hasn't come back (knock on wood).