What are you wearing?
For example in SI there is a amulet (and bit of a challenge to get it) it Disintegrate your armour and weapon to zero.
Just go to effects tab.
If its a game glitch have you try the http://www.tesnexus.com/downloads/file.php?id=9969
I am only level 10 and haven't gone to SI. I have checked what I was wearing and checked via console via the post above and nothing was wrong. My armour is just Mithiral with no enchantments on them. I only have the horse armour mod, none of the others and the unofficial official mod patch didn't change anything.
I am at a total loss as to what is bringing my Marksman skill down by 44, just to say it's nothing in game that is checkable (diseases, enchantments, poisons, etc) all of which are checkable in game.It cannot be an in-game problem as no Dispel clears it as it should. If this info is data held in a file then that file has got corrupted, but in a very strange specific way!
Thanks for the help though - always appreciated!