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That sounds like a bug to me. I would report this to JTAC and consider an upgrade.
I'm going to have to agree with firewall72, and you really should be looking at that upgrade. Even if you don't want to make the leap to 6.0 or 6.2, you should at least be on the latest revision of 5.5 which is R7. R1 releases are NOTORIOUSLY unstable and prone to issues that just weren't caught in QA. I personally never upgrade to an R1 build because I've been burned in the past for those very reasons; R1 builds are very unpredictable.
Talk to a Juniper tech first, but I can almost guarantee you the first thing they'll recommend is upgrading to 5.5R7.