oil pressure warning why????

[WPC80]

Why is my Audi 80 2.8 V6 warning me about oil pressure.
while driving my audi when the revs clear above 3000 rpm on dropping back below this level a sounder is activated and the oil light comes on.
There is no problem. i have measured my oil pressure mechanicaly and it is fine, 75psi at idle and 100 psi at 3000rpm.
so why oh! why do i have this issue.
i have renewed all 3 senders, yes 3 because just behind the crankshaft pully there is a oil temp sender.
i have bridged out senders, shorted them, swapped and replaced them, no joy.
however there are 4 wires 3 senders does anyone out there have the same??
if so is the fourth wire reduntant or is there a place for it? mine has no use that i can see.
Although my car does not have the aux clocks in the console the harness is generic could this be the issue.
i have read loads of info posted on forums buy as yet i can not find a answer to this issue.
Can any one help!
before i adjust the sounder with a large shovel.

[rmac.cab]

My '95 Cabriolet exhibited something similar in the early stages of the problem, so I will describe what was going on just in case.

It started out with the the engine was running the temp light and/or the oil light would randomly light up although the temp gauge showed normal and the oil level was good. After a week or so of being perplexed the engine shut off on 2 occasions and seemed to have a low battery. Various voltage tests varied but the telling thing was that when running the voltage might read <12 volts or >13.5. Obviously something in the charging circuit was intermittent.

Now suspecting a battery or alternator problem I had a local auto parts shop do a batt/alternator test on the car and it did not look right, but did not fail. I began noticing that when the ignition was turned on without starting the engine, ie the dash warning lights should be on, that the battery light sometimes did not light up. Now suspecting the alternator, I removed it and had it tested and it passed. I then took the battery in to be tested and it also passed.

Doing some internet research I found this site: rec.autos.vw [W] TECHNICAL, FREQUENTLY ASKED QUESTION (FAQ) which explained the behavior of the battery charge light and I had my answer. The problem was the the brushes in the alternator were worn enough to make contact intermittently and I was able to replace the voltage regulator which housed the brushes. The vr is integral to the alternator but in this case could be replaced. ~$80 vs ~$200 for the alternator.

From http://www.faqs.org/faqs/autos/vw/technical-faq/:
Q: My charge light does not behave as expected. What is wrong?
A: Taken from EC (March 1994):
The charges light is directly connected to the excitor part of the
alternator. It provides the initial power to get the alt working.
And it also provides alot of additional info:

Engine off, ignition on, no charge indicator:
- Lamp burned out, wiring break, internal alt failure, voltage regulator
bad. Alt will NOT charge! - [From Jens]: Broken rotor wire.
The alternator brushes may be bad which means too short, they should
be at least 5mm out of their housing. If the brushes are wet or
dirty cleaning can help, if not, you have got to change:
o Bosch alternator: change only the brushes.
o Valeo alternator: you must change the complete regulator :-(
o Motorola alternator: the brush carrier has to be changed