flaky streaming video

We "feel" like our PCs are just not able to handle the playback so my question is really about how much ip streaming video can a "normal" PC handle? We have ATI HD 2400 video cards.

On our enterprise network we have 2 vbrick h264 encoders, each with 720p video inputs and streaming video out across our network. We are having very flaky results in trying to view the output video streams on our corporate PCs. We are using either VLC or quicktime to view the streams. The sites have 45mbit/sec of dedicated bandwidth between them and we are using ip multicast so bandwidth is not an issue (testing using cisco ip sla verifies this).

When the PC play these streams they are beautiful. When the PCs don't play they can be fuzzy, pixelated, green filters or simply never start at all.

Encoder input: 720p/60hz 16:9

Encoder output:
- resolution 960x544
- target bit rate of 1.5 Mbit/sec
- transport stream
- ip multicast

thank you for any thoughts on PC capabilities.