Scratch Live Usb Buffer Size

Hi I have owned the NV since about Dec 2014.
I own a Macbook pro 2013, i5 processor, 8 gb ram. Mac OSX 10.10.2.
running latest version of Serato DJ.
Since owning the unit I have found audio will drop out. Sometimes for just a second or less. It also tends to freeze after an hour or so mixing. I have to fully restart the unit and software.
I use DVS and NS7 II I am able to run it at quite low buffer size with those around 2ms generally.
With the NV I have to have it at at least 5ms or audio sounds terrible and crackly.
But even at 5ms still experience the drop outs.
I would rather not set the buffer any higher if possible.
I don't have corrupt files, tried various usb cables and still the problem remains.
It is very random and hard to replicate but I can't trust the unit playing out if its going to cut out even once.
Any ideas on this one?
Live usb xp

Live Usb Xp

Scratch Live Usb Buffer Sizes

2 computers and 4 decks using your favorite mixer rane sl4 for serato scratch live. operator’s manual 2.5.0.