Service seems slow

I have an strange intermittent issue. I have clients calling a flow service via http. Most of the calls from the client return in 300 milliseconds or less, but some calls take 10-30 seconds. When the service calls take 10-30 seconds from the clients perspective, my time of the service execution is still 300 milliseconds or less. I am running IS 6.1 (sp1, fp2) on solaris (sun4u Sun Fire 280R). We have two IS servers fronted by an F5 bigip load balancer. The vitals on the IS look good, cpu is 80% idle, mem use 45%, 99% available threads, very little file I/O.

Has anyone seen something like this before where the service runs really fast but the response back is very slow.

Any thoughts?