After driving for around five minutes, when stopping at a light the oil pressure gauge dips almost to zero and the warning light comes on. If the car is shifted into neutral, the gauge moves up a bit and the light goes out. When driving is resumed, the pressure needle goes back up to a little over 40. Any idea what may be causing this?
The question that you have to answer first is whether you actually have low oil pressure or whether the gauge may be incorrect. You could check the pressure manually with a gauge but it's cheaper just to replace the gauge sender and see if anything changes. If it doesn't help, they you probably need a new oil pump among other things.