Engine won't run

need more info, year,make,model, is the engine light on have you retrieved all engine codes, when turning engine over, is it spinning very freely or just like it always did,
RE POST with all this info. to get a better diagnoses

Problem 5 Engine Cranks But won Start
http://www.linnbenton.edu/auto/perform/no_start.html

Engine cranks but won start

engine cranks but won't run may be caused by either fuel delivery or ignition issue. are you sure when you had the motor rebuilt that the shop put the cam and crank on correct engine timing?

Sluggish engine turnover can be the battery but won't be caused by gas. A problem related to gas would be more apparent after the engine is running.

this sound like new switch.

can you move it by hand?what noise do you hear when trying to start.
is the gear from the starter full in the motor.are all wires connected.

Must not have been much of a mechanic. If the engine runs and sounds fine sometimes and other times it doesn't then it does not sound like a mechanical problem with the engine. Sounds more like a issue with either the ignition system or fuel system.

If it is a 3.8 engine change the crank sensor,they go bad on the 3.8 engines.

Pump or injectors may be bad .

Define - It attempts to turn over but won't catch and run ???? When you turn the key to start does the starter engage an spin the engine ? This would be a crank no start condition . If the starter doesn't engage an spin the engine this would be a no crank no start condition . Sorry to ask but i have only been an auto tech for thirty years an have heard a lot of different discriptions for these simple actions . crank no start or no crank no start . Very different diagnostic procedures . So when you turn the key to start , does the starter engage an spin the engine ,but won't start an run ?

yea got get you more troubles
just check the alternator is charging well and no battery light on dashboard it indicate the bad alternator
monitor your battery if it got old and cannot hold the charge any more
you said when you used another one your car turned on