Try moving the wire around that comes from the high (back) sender.  When my
'86  came home from the dealer after a warranty repair of the head/jacket
seals, my oil buzzer started going off randomly.  It seems they routed the
wire (yellow?) too close to the spark plug wires.  After I routed them away
from the plugs, the trouble stopped.  The good thing about it was it forced
me to install an oil pressure guage up front.  I never had a problem since.
                                                        Jay