I need help starting my 91 Buick Lesabre custom?

[Lotto510]

i have a 91 buick lesabre custom, and it is a hard car to start. we have replaced spark plugs, the coil, battery is still new and alternator, and new mass air flow sensor... It seems like the car starts after a while of not using it, and when im using it i cant turn it off cause it wont start back on for a long time... it turns on every morning when i wake up... but we took it a mechanic and they thought it was the computer so they put a new 1 in and it still is acting the same... i know for sure its not the starter