We're aware of a bug relating to DropDown lists in Reader8.1. I suggest you log this with Adobe - they're aware of the problem, and may provide a fix or a workaround. It may be that your problem is different to the one we know about.
You may want to try setting a default value for the DropDown, this may help.
If you discover a solution, please let all of us know.
Regards,
Howard
http://www.avoka.com