no oil pressure

[Eric Walter <ewalter@domain.elided>]

When I got home last night, my wife said she had to get her '73 Scout
(345)
towed home. Seems she was in town, started it up and had no oil
pressure.  She checked the dipstick, none showing, puts 3 in, now its a
quart high. Oil pressure gauge goes up then falls back to zero. I get
home, start it up, pressure is fine.  Today she said the pressure was
fine when she started it, then slowly dropped to zero on her way to
work.
So, is the stock gauge mechanical or electrical? If its electric, maybe
its just a loose connection?  I'm assuming that any mechanical failure
that would cause the pressure to drop to zero would be permanent. 
Unless the pickup tube fell off 2 quarts low shouldn't cause the
pressure to drop to 0.
Any help appreciated,
Eric