The Jetta's have the same problems with their Dynamic Oil Pressure system
as the the '86+ Vanagons.  I would have the oil pressure checked by a
mechanic or install a guage--real easy on the the Jetta.  My '89 diesel did
the same thing.  I put a guage in to verify the oil pressure.  If the
pressure is OK, the fix is to replace something on the back of the
instrument panel.  The buzzer usually goes off after exiting the interstate
on warm or hot days.  Check it out with a guage before you drive to much
more.
                                                        Jay
                                                        '85 Westy
                                                        '89 Jetta