Co-channel interference from WMPT @ 234° is an obvious concern. Don't rule out FM interference either, you can check your FM Fool report for free, and add an FM trap for relatively little cost. The FM trap sold at Radio Shack is fine, and probably the easiest and least expensive next step available.
After that, you can try alternate mounting height and location.
Failing all that, stagger stacked antennas may be successful.
http://forum.tvfool.com/showthread.php?t=1024 This would require some fabricating skills on your part, and some willingness to experiment.