The order doesn't seem to matter: reversing the order of questionnaires within the experiment doesn't fix the freeze, but reducing the number of items does. (I can send the "Test Experiment" I developed back-channel, but the scales can't be posted in a public forum.)
It is indeed odd to find this on a stripped-down XP SP2 machine with basically nothing on it as well as on a Windows 7 Enterprise machine that's stuffed with software.
Are any other ML users having success with long experiments?