Have you had a mechanical gage put on it?  Perhaps there is an
intermittent electrical problem between the sender and the gage (perhaps
a corroded and/or loose connector terminal) or in the gage itself.  The
mechanical gage would bypass such possible problems and confirm whether
the pressure itself is truly fluctuating or if it is erroneous gage
readings.  If you aren't getting any indication from the engine that the
pressure is truly dropping to zero (valvetrain clatter, etc.), you might
just assume an electrical cabling or gage problem and troubleshoot it
accordingly.

Bill Putney
(To reply by e-mail, replace the last letter of the alphabet in my
address with the letter 'x')