1999 E350 7.3 diesel, 147k miles, always garaged, hooked to double battery tender, and well maintained. A couple of years ago, I modded the fuel tank, installed an Air Dog, and upgraded the fuel pressure regulator in the Fuel Bowl. Fuel pressure was 67, dropped to 60 when pulling out or romping on it, but would quickly return to 67. Since May, I've had to be away and have returned home 3 times in the last 4 months; the van has been started 3 times. Last month, it was hard to start, slow long cranks and the fuel pressure was 58, which zeroed when pulling out or romping. Pressure is slow to recover, sometimes only to about 50. Called Air Dog Tech, and he recommended changing the fuel filters. I did that this time, but no improvement. Am I looking at an HPOP problem? What can anyone recommend for troubleshooting?