OK! I replaced the fuel pressure regulator. I have a spare SPFI manifold I traded for the carburetor one I had on my 86 GL. Took the regulator off that spare one, which looks like it wasn't very old / low miles /indoors most of it's life.
Car fired right up. Pressure nice and steady, at 24PSI - which is a little high. Drive to the bottom of my safest "test" hill. Checked the vacuum line to the bottom of the pressure regulator, oops, I left it off. Connected. Pressure more normal 22PSI. Still nice and steady. Drive up the hill, NO PROBLEM! Loop around for another test, and no problem. Took a run up the steeper bigger higher speed hill nearby, and no problem.
Pressure now at 20PSI, but it is running fine as it always has.