Why wont my car crank over?

Hi guys,
Have recently noticed that my car wont start when trying to crank over...
I find this strange as i replaced my battery about 2 months ago.
My initial thoughts were either a faulty new battery or a rooted alternator.
So, out comes the multimeter and did a few testings....
With the car running the alternator showed a reading off 14.0 Volts, this was with all electrical components (Heater, Stereo with 2 Amps and 12inch Sub, Headlights, Wipers) all switched on. At the same time I checked the battery which was showing about 14.2Volts so there wasnt much difference..
A few ppl have said to try disconnecting the Battery positive terminal while the car's running and if the car switches off on its own then its a faulty Alternator but mine seemed fine.
After jump starting my car with my other BA the car was running fine, actually took it for a spin and all was well but as soon as i switched it off after 15mins of driving than there was a no go? So i go and check the battery with my mutimeter and the volts suddendly dropped to about 11.2volts? I then change batteries with my other BA as i thought maybe it was rooted but i had the same issue with that connected as it still wouldnt crank over....(and showing roughly same volts 11.4)
Im not a pro but it seems with some of the testing done that the alternator and battery are working fine as they're both showing 14+ volts while the cars running so im kinda puzzled what the issue could be as ive checked for loose connections and all seem to be intact....
btw cars a ba xr6t
Any advise would greatly be appreciated.
cheers
Have recently noticed that my car wont start when trying to crank over...
I find this strange as i replaced my battery about 2 months ago.
My initial thoughts were either a faulty new battery or a rooted alternator.
So, out comes the multimeter and did a few testings....
With the car running the alternator showed a reading off 14.0 Volts, this was with all electrical components (Heater, Stereo with 2 Amps and 12inch Sub, Headlights, Wipers) all switched on. At the same time I checked the battery which was showing about 14.2Volts so there wasnt much difference..
A few ppl have said to try disconnecting the Battery positive terminal while the car's running and if the car switches off on its own then its a faulty Alternator but mine seemed fine.
After jump starting my car with my other BA the car was running fine, actually took it for a spin and all was well but as soon as i switched it off after 15mins of driving than there was a no go? So i go and check the battery with my mutimeter and the volts suddendly dropped to about 11.2volts? I then change batteries with my other BA as i thought maybe it was rooted but i had the same issue with that connected as it still wouldnt crank over....(and showing roughly same volts 11.4)
Im not a pro but it seems with some of the testing done that the alternator and battery are working fine as they're both showing 14+ volts while the cars running so im kinda puzzled what the issue could be as ive checked for loose connections and all seem to be intact....
btw cars a ba xr6t
Any advise would greatly be appreciated.
cheers