In reply to Phil Miles.

Hi Phil,
My rate in the lab we about 30 devices sending about 4 flow records each per second. So, about 120 total records per second and I didn't see Logstash act up. I am not sure about the rate that your Netflow exporter is sending, but probably it is much more. Also, I can't be sure if Logstash crashes due to overload. If Logstash crashes for you consistently maybe you can check their mailing list http://www.elasticsearch.com/community/ I am sorry I couldn't help more.
Panos