My 2003 Monte only starts when it wants to

I am new to this so I am sure someone has already had this problem.... I have a 2003 Monte Carlo LS 3.4 and it has been giving me problems for awhile now. In the morning when its cool outside, it will start just fine, but after work when it is sitting in the sun, it will not start. I can usually tell when I try to unlock it with the remote and nothing happens, or when i open the door and notice the dome light isn't on.I turn the key and nothing:disappoin
The radio doesn't work door locks don't work but windows roll down.
The message center flashes battery, security, and service vehicle soon..and the check engine light stays on.
My ex who is or "was" a mechanic on older vehicles ran a push button to the starter to try when it doesn't start. well it turns over with the button and only runs for like 5 seconds and dies. Yesterday it would not start all day long and this morning it fired right up. I need my car to go to work, it is the only vehicle I have and being a single working mom is almost impossible without a car... PLEASE...Can anyone help me????

Check for a bad connection at the connectors along the frame down on the right side by the battery. They can get corroded and cause all your problems that you are describing.

this appears to be a ground problem as too many electrical devices are failing and also the fact that it is intermittent....

this could also be a bad battery I have seen internal battery failures where the internal connections where broken,,,usually after an impact....

get the battery load tested if all the connections positive and ground are clean and secure....