Strange Latency Issue Netscreen 50

Hi,

We're having a bit of a strange issue with latency and can't figure out why.

We have two Juniper Netscreen 50's running in HA mode at the gateway of our network. Every Wednesday the latency shoots up to almost unusable levels and remains that way until the firewalls are rebooted. Once rebooted the latency drops back to normal and remains that way until the next Wednesday.

Any ideas what could be causing this? Or any commands I can run to investigate and provide more information?

Thanks