Comment 8 for bug 231171

Ok, the md5sum will definately not check out with the version you are trying to install. Your sources.list does suggest you have gutsy-updates enabled though. Could you try with another mirror? (System - > Administration -> Software Sources, then select another location in 'Download from').

If that still doesn't work, could you select the flashplugin-nonfree package in Synaptic, then go to Package -> Properties and select the 'Versions' tab. Could you tell me what versions are shown there? You should see the old version from gutsy and the more recent one from gutsy-updates.