Car sometimes won't start as if out of gas or spark. 50 amp. boost charge will start car, jump start won't. Jump start did not work today but an hour later car started when I turned the key, Any ideas what to check, Battery and Alt. good. Greatful for any ideas.

 

Welcome!
This area is for website questions, so I'll move this to General Tech where you're more likely to get an answer.

Check BOTH ends of all battery cables. Sometimes they can be corroded or loose at the other end from the battery, & that can cause problems.

 

This Could be a main relay problem.
The typical scene is that the car will start in the morning but not after sitting in the sun all day. Hot interior/no start..............Cool interior/YES start.
You can pop the guts out of the relay housing up under the dash, and resolder the relay coil joints on the circuit board.
Beyond that diagnostic, it is best to find out if you are losing spark or fuel pressure.
You should be able to hear the fuel pump run up presssure for 3 seconds when the key is turned on.
Spark check by using an old spark plug to see if there is spark going out to the cylinders.