There do seem to be a few bugs at the mo, although I don't get jumping issues on the app myself.
As an alternative until the player is improved if you have Silverlight installed on your machine, you can still use Firefox to watch in-browser via Silverlight (if you don't have Silverlight installed Firefox also prompts to install the app).
Hi there here is some more information we have regarding the Mac/ silverlight issues, hope this helps..
The NOW TV Player has been created for Mac users following the withdrawal of Silverlight
Microsoft has now withdrawn Silverlight support for Macs, so if customers don’t already have Silverlight installed, they’ll no longer be able to download it. NOW TV have create an alternative way of viewing for Mac users the NOW TV Player
How to install the NOW TV Player
To trigger the Player download, you’ll need to choose something to watch and hit play. You’ll then be prompted to download the Player.
1.Restart the NOW TV Player Close the NOW TV Player, go back to the NOW TV website and re-launch the NOW TV Player by clicking play on the show, movie or sports program again.
2.Reinstall the NOW TV Player
3.If the customer's unable to download the NOW TV Player file, send them this link to direct download it web.static.nowtv.com/watch/NOWTVPlayerInstaller.zip
More details on how to install and troubleshoot the player for Mac can be found on iKnow2, look for "NOW TV Player for Mac"