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What happens when it doesn't start? The fact that it sounds like the problem was originally happening after sitting overnight leads me to believe that the engine cranks but doesn't start when this happens.
If that's the case then the first thing you'll want to do is check for fault codes. If you don't have a code reader or scan tool of your own you can have the codes read at most large parts stores like Autozone for free.
If there are no codes stored then the next thing I would recommend is checking for fuel pressure bleedoff. If fuel pressure bleeds off through the pump or a leaking injector as the car sits then it will cause a long crank time. This would start when the car sits for longer periods of time and would progress to happening if the engine is shut off for just a few minutes.