Wow, i was just about to make a thread about this issue.
My car 98 Z/28 does the same thing. The oil gauge can be reading normal then all of a sudden when i am at a light it will start to flutter and it goes down to about 20 psi but stay around 40 while cruising.
To add to this, my cars timing seems to be off, theirs a small shake when idling at a light. Its like a hick-up, anyone think it could be the spark plugs? I am thinking of changing the plugs and wires.
I've been told this is normal for an aging pressure sender. As long as it reads 40 at a cruise and at least 10 psi for every 1000 RPM, you should be all set. That's the hand-me-down wisdom I've received anyway.