Possibly the machine buffer is running out of packets. This can be caused by excessive lost packets and/or lattency on the network links. Is it on a WAN link? If so look at distributing the video (if on demand) out to the edge. IP/TV servers supp...
Try implementing the Cisco MCM, it will prioritize H.323 traffic onthe links.If you do not have a pure Cisco network look at CSU from ADC that support Service point, they can filter and prioritize packets on the WAN links.
UDP packets for Audio are normally 480bytes and a max of 1469 (UDP packet size).UDP packets for video are a max also of 1469bytes but can be as little a 40bytes. Video packet size varies dependent on video motion. TCP packet sizes depend on the messa...