2002 Sierra Dies (seems like not getting fuel)

2002 GMC SIERRA 2wd 5.3L
A couple weeks ago, it began to idle rough and is difficult to stay running. Now it will turn over then die. Still idles rough and drops to about 600 rpm at idle.
The Service Engine Soon light cam on, so I hooked it up to a diagnostic scanner, which showed code P0300 "random/multiple cylinder misfire detected". I have replaced the fuel filter and spark plugs (the center electrodes were pretty worn, but all cylinders looked like they were burning good), but the problem still persists. I don't see any vacuum or other hoses unhooked or cracked. The symptoms seem to be similar to a choke malfunction on a carburated engine.
Whats wrong?
How can I fix it?
thanks,
Tony