Rare variable name corruption

I'm having a problem with a rails app occasionally getting a variable name corrupted. For example, a form field person[last_name] may be sent to the browser as person[last_9ame]. Or a similarly corrupted field name may be sent in a SQL query. Restarting the affected process solves the problem.

I'm running Rails 1.2.3, Ruby 1.8.6, and Mongrel on 64 bit Ubuntu 7.04.

I suppose bad memory is a possibility but before I put this machine in production, I ran MemTest for about a week with no errors. I'm also not having problems with any other processes on server. Also I would think that bad memory would be at least as likely to crash the ruby/mongrel process as to always corrupt a variable name, and I'm not having any problem with my ruby/mongrel processes crashing.

The problem only occurs every month or two, and I don't have anyway to duplicate it.

Any ideas?