Sounds like memory corruption. That causes all sorts of things to go wrong.
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Have you tried doing a build (from the Tcl sources, instead of using someone else's build) with memory debugging turned on (<tt>--enable-symbols=mem</tt> option during <tt>configure</tt>)? Or have you tried using a tool like purify/efence (whichever you've got) to spot where things are going haywire?