Check the oil pump relief valve spring. It might be getting weak as it heats up, and/or the relief valve could be sticking. Polish it, and the bore it runs in with jeweler's rouge and a hard felt buffing wheel.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!