This issue that you are experiencing with the video on your Mac is very odd; you already confirmed that you do not have the same behavior using a mobile, so this is not an issue with the Slingbox itself. Probably, there is a conflict with the browser that you are using; have you tried to reset the browser to factory settings?
I also recommend you to try a different browser to confirm if the issue persists. If you are using Safari, try downloading Firefox and report back the results. Have a look at this article: