Engine Performance problem
1999 GMC Jimmy 6 cyl Four Wheel Drive Automatic 76000 miles

I have had a very intermittent starting problem with my Jimmy for a couple of years which has recently become more frequent. It doesn't matter if the truck is hot or cold. There doesn't seem to be anything common except that it just suddenly decides to start up just before the battery goes completely dead from cranking. It was suggested that it could be the fuel pump relay which I swapped with the AC relay during the last occurence but it had no effect. A few tries later, it just started again.
Then the next day, a short local trip, a five minute shutdown and it happened again. Just a note: it seems to catch for a second and the tach goes to about 1000rpm's and just shuts down as if I had just shut off the key. Any troubleshooting advice would be appreciated.

Check the fuel pressure to rule out the fuel pump and then comeback

I checked the fuel pressure tonight. With the key in the on position the pressure is 62psi. I also tested it running. It's 55psi.
I have no running problems. Just intermittent starting.
I also borrowed a Snap=On code reader and found no stored engine codes.

Next time it shutsdown immediately look for spark-no spark check the cam and crank sensors.

Wouldn't a malfunctioning cam or crank sensor generate a code? I'm not sure. Just asking.

I'm sorry. I forgot to ask. How do I check the cam and crank sensors short of replacing them if their failure doesn't generate a computer error code. I'm trying to avoid the costly "parts swapping" method of troubleshooting.

Sometimes a sensor/s failure wil not generate a code. Check the computer with and w/o a code/s, you never know what will show up.

I gave you both just in case it has no spark-causing the intermittent start-could be very well be the computer that's acting up-who knows

Check its resistances/gap