First, do you have the build that was released last month, or the build released in April? If the latter (April) please install the latest build available from Connect.
Assuming you've got the latest build, you might want to tell us more about your media files (codec, etc.) But if it's the issue I think it is, it's already bugged, and I've been beating Microsoft up about it for weeks. But for the time being, don't use Chrome
or Opera. Use Internet Exploder or Firefox/Mozilla and (for Firefox) don't use the beta version, stick with the released version.
There is, unfortunately, a pretty good chance that Microsoft will eventually say "This is an external issue", and close the bug as "won't fix". Which is kind of stupid, since I'm pretty sure the "bug" is in the Silverlight player or the streaming server
on the head end rather than the browser (it's just that the combination of Silverlight and browser lets the bug manifest itself), but there's not a lot we can do about it but threaten to hold our collective breath...
I'm not on the WHS team, I just post a lot. :)