Hi,
A few recommendations I think might help.
First, try upgrading to the most recent 4.0 release. This is always the first thing I recommend, there are lots of possible bugs fixed in each release.
Second, try repeating this on another machine. Very commonly repeated table corrupts are caused by hardware and/or OS level problems (such as drivers, etc...) If it doesn't occur on the second machine, then it sounds like a problem with the machine at some level.
Third, if this repeatable on the most recent 4.0 release and on multiple machines, then I would file a bug at
http://bugs.mysql.com and give them the data to work with, so that they can fix the problem (since at this point it sounds like a mysqld bug)
Harrison Fisk, Trainer and Consultant
MySQL AB, www.mysql.com