The answer is the same for pretty much any vehicle.  Either:
(1) Your pressure is in fact low, and you need to find out why and get
it fixed, or
(2) The pressure sending unit is out of calibration and not reading
correctly.  Sometimes this is due to a leak internal to the sending unit
- and the condition can vary with temperature and the degree of leaking
and internal presure throwing the reading off from day to day - which
would go along with the inconsistency of the problen that you are seeing.

Hopefully it's (2) - and chances are good that it is.  You can do one of
two things:
(1) Have the pressure checked - actually measured with a known accurate
gage attached to the pressure port, then go from there depending on what
you find (i.e., if true pressure readings are fine under *all*
conditions including the stopped-at-light one, then you know it's the
sending unit) or
(2) Replace the sending unit and see if the problem goes away.

There is a very good chance that it is just the sending unit.  If you
simply go ahead and replace the sending unit, it might be advisable to
have the pressure readings done anyway just to make sure the new sending
unit is not wrong and giving you a false sense of security.  The extra
you spend for the pressure reading with a gage would be considered
insurance against ruining your engine.

The choices are yours.

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