How would you track down latency jitter?

I'm currently in a finger pointing match with another ISP. Lets call them Jitternet. Were seeing extreme levels of out of order UDP packets which are just dropped by our application and are essentially lost packets. The nature of the application prevents doing any kind of jitter buffer and we just have to take the data as it arrives. I would have thought providing the data that the 5 other major ISPs we peer with do not show this problem and peer at the same location would have been sufficient to prove the issue isn't on our side but Jitternet disagrees. We also peer with Jitternet at a secondary location and clients behind that peering show much better performance.

What testing could I run that would track down the source of latency jitter on udp streams? MTRs don't seem to show any problem because the issue only seems to arise when a small burst of data is transmitted (5KB in a few ms), not just on smooth datastreams.