wp_posts table 'in use', 'repair' fixes it, but 'optimize' crashes it

I have a periodic and ongoing problem that seems to be in the wp_posts table. It seems to crash (indicated in myPHPAdmin as “in use”) from time to time. A “repair” gets it running again, but if I try to “optimize” the table it crashes (“in use”), no content shows at my site, and a repair fixes it once again.

It is a big site with lots of old content —?I’ve used wordpress for a long time and imported content from other older content management systems — and I really would like to optimize this table.

Thanks for suggestions.

Dan