Oil Pressure Warning Light 1.6TD

[smk0407]

I'm having a problem with the dashboard oil pressure warning light on my 89 1.6TD T25. It comes on when the van is revved, and then stays in 'flashing' mode. I have the occassional problem with the buzzer when the van has been running at high revs and there is a sudden drop in speed. I've had both of these problems for years and the van is running fine, so I do not suspect there are serious problems with oil pressure and I'm going on the assumption there is a fault in the wiring or one of the switches.
I think I have the correct upper pressure switch intalled (the black one). I'm not sure about the other switch (I believe there is another one lower on the engine somewhere?), so the fault could be with that one.
I have to pass a road worthiness test, and it will fail if I do not fix the flashing light problem.

Does anyone have a suggestion to confirm what the actual problem is, or a quick workaround that will ensure the light does not come on when revved?

Many thanks in advance.