oil pressure warning light question - 5000S

[JAUST@tandb.com (JRA)]

My 84 5000s has just started exhibiting a disturbing trait:

After running 15-20 minutes at freeway speed, the oil pressure
warning comes on - complete with warning chime, then goes off,
and repeats the process every few minutes after that.

Temp does not rise, the oil level is ok, and the oil itself is
only 2,500 mi old - semi syn castrol.

Does anyone have a clue as to what might be going on?

Jim Austin