Hey dark soul... sorry to hear about the plec, and that you're having a rough time otherwise... best wishes...
I've lost two plecos in my tank previously, and they went in a very similar fashion. The first died within 24 hours from the fish store, and could have been sick already, or I could have acclimatized him too quickly. The second, however, lived in my tank for awhile, till suddenly he did the exactly same thing yours did. I traced it to a ph shift... the change in the ph was so stressful for him that that was what did him in. I've finally decided to get another one and so far he seems really well, but I'm nervous.. they seem very sensitive to me. (I don't have anything in my tank that ever goes near him... except for the cories, who just seem to go right past him and he could care less!)
-zt.