20 seconds for oil pressure light to go out ??

Hi folks, I am both confused and concerned When starting from cold my 1999 TD5 oil pressure light takes a good 20 seconds to extinguish but then goes off, it comes on intermittently when driving but with no real pattern to it, it is just now and then but mostly it stays off.
I would have thought that if my oil pressure is low the light would be flickering on tickover and then go out when the revs increase?
I know there are issues with some oil pump bolts but my car has 150,000 miles so I am assuming it has been done or is ok?
I have changed the oil pressure switch to no avail.

Does anyone have any thoughts?

Thanks,Will.

Which engine is it?

It could be a mix of electrical problem with the sensor , bad earths or pump shafted, if it is flashing up intermittently like you say then I would veer more towards electrical issue, check all the wires especially where they go through the bulk head as there is a good Chance a few have lost insulation and be causing the prob