999 added a post in a topic Win7 on HTPC stuttering A/V through HDMI
I've had some issues playing HD with my Radeon HD 4300 & Pentium 4. I was able to play HD videos well by disabling GPU acceleration with VLC (you might have it enabled).
I dont think the 8300 has a separate audio chip. I think it would show in the device manager if it does have an HDMI audio driver.
Any chance you can try DVI?