Iperf works really well:
http://dast.nlanr.net/Projects/Iperf/
It will do tcp/udp/multicast; you can pick the rate on
the udp side and the client/server architecture lets you
measure jitter, out of order packets, loss, etc.
So you could load up a fixed rate of udp and then produce
your burst using a second copy, etc. to see what happens.
David