Have your transmission guy check the fit of the pressure regulator valve in the main pump housing. I'll bet he finds a score mark, or possibly just a very tight fit that allows the valve to swell and stick in the open (pressure relief) position after running a few miles and heating up. Polishing the valve housing with some jeweler's rouge and a soft cloth wrapped around a dowel rod in a variable-speed electric drill motor usually solves the problem.
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!