I'm having a (possibly) similar issue:
- shared Linux
- WP 2.1
- intermittent 500 Internal Server Errors
- happening in various themes (I use MistyLook)
My issues that are not similar:
- usually only occurring on home page
My issues that may or may not be like yours:
- error occurs the same in FF and IE (have not tested other browsers)
If you change your Home Page to something other than most recent posts, do you still get the error? When I do that, my error immediately & completely disappears. Only thing I've been able to figure out so far, is the error seems to be tied to publishing posts to Categories (Tags) - either certain ones or ones that are nested 3-deep (Example nested hierarchy: Food > Lunch > Sandwiches).
(1) change my newest ("offending") post to draft AND
(2) make my Home Page show most recent posts again
the Home page is fixed.
Also when I
(1) change categories for my new post (like only post to categories/tags that are 1-category deep) AND
(2) republish my new post AND
(3) make my Home Page show most recent posts the Home Page is fixed.
Not sure if I'm on the right track or not, but for now this seems to be the fix for me. I'm keeping track of my intermittent 500 Internal Server Errors and the fixes here.