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How about running Xenu LinkSleuth over that part of the site to see what you get?
Are there any canonical issues with the URLs? Is there a problem with parameter strings being too long, or with parameters being in a different order depending on the link that was clicked.
This needs a very careful look under every rock to see what might be causing it.
I have no idea if this is normal or not. It is a soho Fantastico generated php website.
I run one of the php pages through Link Sleuth and it seems to run fine.. duration 3 seconds - 43779 size - out links 141 - in links 79 - text/html type - status ok - Apache Server.