Get your MSL and read this :
Streaming media and the Tyrrany of the Sprint Proxies
Spirit uses proxy servers and many video streams pass through this server before it hits your device, reducing quality and causing some buffering issues in some cases.
It's in the Evo 3D forum but the info works for this device.
This was the first mod I did when I got the device.
EM is the author of the thread and his posts and information are sound.