Herb,
It's just my crackpot opinion that air in the gage line serves as a pressure buffer between the oil & the gage. Air is compressible, oil isn't. Try loosening both ends of the gage line to let a little oil run out of the line & be replaced by air. Tighten the connections again & see what happens. The actual pressure pulsation is likely caused by the pressure relief valve rapidly seating & unseating. It's possible that this vibration could be reduced by putting a 1/8" shim under the relief valve spring. Or replacing the valve & spring.
D