Internal server 500 error with 958 records (not maxRequestLength issue)

Internal server 500 error with 958 records (not maxRequestLength issue)

I have a System.Web.UI.WebControls.GridView control. This is running in an
ASP.Net SharePoint Web Part application using .Net 3.5.
On form submit, if this grid has 957 rows, it works fine. If it has 958
rows, it fails. I'm confident that this is not a maxRequestLength overflow
because I have changed that in web.config, and I've also loaded 957
records of large size, and then 958 records of a smaller size and the same
behavior occurs. It's not any one record either because I load more
records than that and then randomly eliminate them before returning the
view to the client (which is how I found the magic number of 958).
It's never random; 957 records will always submit no matter what the total
size and despite any delays while debugging (no timeout occurs). 958 will
always fail. The actual server error returned is:
Uncaught Sys.WebForms.PageRequestManagerServerErrorException:
Sys.WebForms.PageRequestManagerServerErrorException: An unknown error
occurred while processing the request on the server. The status code
returned from the server was: 500
I am beyond stumped. Has anyone run into something like this before or
have any ideas?