1998 GMC C2500 Fuel pump? Sensor?

Vehicle will spin, somwetimes it will start and then die a second later. Sometimes will not fire at all. Relay is working, pump is whining. I checked fuel pressure at motor, it squirts out pretty good. Do not have a guage though to check actual psi. It has done this several times over the last year. Usually it will sit for a while then, retry and it will start ok. This time it hasnt run for several days. The security light on the dash comes on occasionally lately, and the service engine soon light came on for a couple of days but now is off. I have changed pumps in it twice. The last about 30,000 miles ago. I know what a pain they are, but this time it acts differently. And, it starts working again for several months before it acts up again. I think it is a sensor maybe, but do not know where to start looking.