Strange oil pressure indication in GTV 2.0?

[Einar Sjaavik <einar@ski.mail.telia.com>]

Here's one for you dyed in Alfa fluids.

My oil gauge seems to act as it should up to 2000RPM,
rising with RPM up to 4kg/cm2 with a hot engine. Then it falls off 
all the way down to 0 as the RPM rises!? If I depress
the clutch, the pressure comes back. But I then also
let it fall below 2000RPM.

Here's datapoints that I can imagine may be of significance:

- Engine has 190.000km, but sounds fine.
- It had a head overhaul just before I bought it according to PO.
  I don' know why.
- Dipstick is long enough.
- No sign of contaminated oil.
- No sign of contaminated coolant.
- Speedometer works only now and then. It seems it will
  decide upon working or not as I turn on the ignition.
  I does not flick between failure and working when driving.
- I cannot tell if this fault has come suddenly or not,
  as I have only driven some 30km since I got the car.
- Ohter instuments and lights works now. But only after I 
  undid some funky wiring introduced by PO.
- The gauge never falls below 0 like it does when ignition
  is off.
- I thought the pressure rose as I depressed the clutch,
  but I'm not so sure anymore. I now believe it is RPM dependant.
  With the clutch at the other end of the car, should there
  be any fault that could make clutch position a factor? If it 
  was at the crank, I would guessed at worn thrust bearings.

While in lurk mode I have seen mentioned that the pressure
sensor is faulty in many cases. But how can it err synchronous
with RPM or the position of the clutch?

I will if necessry install a manometer type instrument to see
if the pressure is really falling like this, but if this
is a known mode of failure, please tell me. It might save 
me the plumbing.
I'll be driving carefully and mostly <2000RPM to map any
dependencies I may have overloooked.

-- 
Einar Sjaavik, einar_sjavik@login.eunet.no,
http://www.login.eunet.no/~einar/
1986 Alfa GTV 2.0, 1986 Maserati Spyder 2.5, 1974 Mini 1000, 1952
Ferguson TEA