I have the same problem when using NowTV Player. Have contacted them via Twitter to which their only response was "Have you tried a reboot?"!! Oddly, this issue only seems to present itself when watching 'live' TV, as when pre-recorded items are shown (such as adverts), the sync seems fine.
Alas, It seems NowTV are really not interested in addressing this problem, and simply make the assumption that it's the end user that is to blame, and not their player.
It should also be noted that if I play through Internet Explorer (with Silverlight), everything is perfect. However, I don't care for IE or, indeed, Silverlight which, in any event, is due to be discontinued in the future. As far as I can see the only real answer is for NowTV to completely review their player and update accordingly - but will not hold my breath! In the meantime, my purchasing of passes is reducing at a rate of knots!
I have the same issue with the player. Starts off OK then slowly goes out of synch, but not always. Haven't used live TV, but happens with box sets or movies. I find that if I pause the player and then hit play again it resynchs itself and then slowly drifts again. At least that gets rid of the problem for a little while.
I'm using version 126.96.36.199 on a windows 10 laptop.
Come on Now TV. It seems people have been complaining about this on this forum for over a year and no credible response from you so far!