Horde webmail problem

[icg_Rob]

This is in regards to one of our customers who is using webmail, which is run on a software package named Horde (included with Plesk but also available as open source). When the customer uses webmail sometimes the CPU usage for one or more httpd processes will spike to 25% of the CPU and stay there, locked up, whether or not the customer is still using webmail. From the customer's side, the webmail interface just sits and hangs forever. It's been duplicated in IE and Firefox so I don't believe it to be client side, though it could be related to something in the mailbox.

We're running Plesk 7.5.4 on RHEL3 (Red Hat Enterprise Linux version 3).

Has anyone else encountered this problem or have an idea of what the problem is?

Thanks,
Rob