2000 Chevy Silverado 1500 5.3

My truck has random issues starting. It sometimes starts and works but when
it is off sometimes it does not start for hours or a day or two. I have changed
the fuel pump 3 times,fuel filter, fuel regulator,spark plugs, spark plug wires,
fixed all vacuum leaks that I know of, cleaned intake and throttlebody. What I
notice when it cranks is that when checking the valve on the fuel rails I
sometimes have zero fuel in there. When it runs right it has fuel squirt out
when pressed. I have taken to 2 shops and dealer and they all tell me the
truck runs great. Dealer wants to replace intake manifold to possibly stop
random misfire codes and lean in bank 1&2....possibly. The first shop blew
out the fuel lines and it ran good for a few weeks then same symptoms came
up. What can cause no fuel pressure at the rails when key is in the on
position? once I get fuel in the rails my truck runs like a champ. Can anyone
help me out....this has been going on for close to a year now....thanks