So its not just mkv files although uninstalling all java applications and reinstalling the latest java seems to have fixed it, ill keep testing and keep posting my updates, but im not indoors that often nowadays, its summer!!!
danse, i'm not sure if what i did in terms of inbound and outbound rules is required, if you can see and stream media on your PS3 then i think you should be fine, I would try uninstalling all Java then reinstall the latest Java, hope that helps!
Some brief instructions Click start, tyep in firewall and press enter, choose top left inbound rules, create new rule for port, port is 5001 and name it accordingly. Repeat for port 1900 and repeat both for outbound rules. I also created a third rule that was associated to a program and selected PMS.exe.
You will also have to do this on your router or any other personal firewall you may have going on your system.
Here is a link provided in the PS3 Media Server Tutorial/FAQ sticky in this forum http://otmanix.de/english/2009/05/17/java-ps3-media-server-for-dummies-chapter-2-networking-and-technical-basics/