I'm wondering maybe if this doesn't have anything to do with the PIX configuration at all, but maybe rather my router.

I bet you're right. We had a very similar problem with a PIX 501 that we installed a little over a year ago. For some reason it wasn't playing well with the routers, so we solved the problem by adding static routes for our ip block to the pix outside interface. You might want to give that a try and see what happens.

- trs