Why would a consumer web part get interface data from a provider but
only on every other post-back of a page?

I have a dropdown WP and an SPGridView WP. The dropdown is a provider
and the SPGridView is a consumer. If I make a selection in the
dropdown, the SPGridView is populated with the correct data. If I
make a different selection in the dropdown, the SPGridView WP receives
a null provider interface. I'm wondering if I've got an issue in the
order in which things happen on the page. I've debugged this and on
every-other load of the page, the SPGridView WP receives a valid value
from the provider. Any suggestions on how to pinpoint this problem?