SimpleMarket Sidebar and Nav problems in IE8

I am running the SimpleMarket theme along with Marketpress, Affiliates and Membership plug-ins. Everything functions correctly in Chrome, Firefox, Safari and IE9 but in IE8, the left sidebar drops to below the body and the submenus in the top nave, though visible, can't be clicked on.
(running Wordpress 3.6, Marketpress 2.8.8, Membership, and Affiliates

I tried a clean install of Wordpress and the theme (without all the other plugins) and still had the same results in IE8 - funky sidebar placement and non-functioning sub-menu in the nav.
(running Wordpress 3.6.1, Marketpress 2.8.8, Membership, and Affiliates

(Since then, I have also added in the Marketplace and Membership plug-in to the test site with no apparent changes.)

I have deactivated the them and other plug-ins. I have had plenty of experience modifying php pages but I don't write it so some solutions that probably should be obvious to me aren't. However, I have searched through countless lines of code and compared to other themes with a left sidebar and drop-down nav menus but I can't find the problem.

I didn't see anything in the Community forum that already answered this. If I've missed it, please send me a link. Thank you!!!