Chevy Truck with problems starting

My 1999 Chevy Silverado is having trouble starting. I know it's not the battery, and I just replaced the fuel pump 4 months ago. It sounds like it wants to start but when I let off the ignition it dies. This happens if I haven't started it in a few days. Oddly enough, it starts just fine in cold weather but not when it's warmer. Is it a loose cable or what? I don't want to take it into a shop and pay $1000 for an easily replaceable cable. What could it be?

Thanks to all

Greetings Jables welcome to the Forum.

I'm thinking it more like an Ignition problem, loose/worn contacts in it, which are not maintaining you circuit, when you release it from start.
My best guess.

Have a good Day !
S.W.

If this truck has sensors on the air intake tube between the air filter box and the intake. You could have a bad mass air flow sensor or mapp sensor that only likes to work when its cold out. Sometimes these sensors can be checked by auto zone or advanced auto parts.

1. Use a multimeter and check the ground by using the lowest ohms setting place one of the leads on the Negative battery terminal/cable and the other lead on the Negative battery post. The multimeter should read 0.00 or really close to it.

Check the battery terminals by trying to twist them in either direction, look for corrosion, remove the terminals and clean with a battery terminal cleaning tool.

2. With your multimeter on the ohms setting put one lead on the negative battery post (with everything connected) and the other lead on the vehicle body (use a bolt head some where). This reading should be 0.00 also. the lead on the vehicle boy can now be moved to a bolt on the engine block (you can use different bolts if necessary). This reading should be 0.00.

3. With the multimeter on the lowest ohm scale place one of the leads on the positive battery terminal/cable and the other on the battery post. again looking for 0.00. place one of the leads on the positive battery post and the other lead on the big post on the starter where the positive battery cable connects to the starter, you should read 0.00. With the meter on volts scale at or above 12 volts check for 12 volts at the starter, check for 12 volts between the pos. batt. terminal and the vehicle body and the engine block.

4. Take the ignition module to the parts store to be checked on their machine. Look at the distributor cap and rotor button for excessive wear, cracks, white streaks on the distributor cap, dirt, etc.
Check the vacuum hoses for cracks (warm parts swell and cold parts shrink). gasket leaks on the intake, throttle body, etc.