Kaz: try usual methods: reinstalling and patching. It sounds like the file that contains the animation of the wyvern dying is corrupted or something.
About wyvern heads, i havent played in a while (now im playing again from time to time but i havent got that far), but i think Thalanthyr from High Hedge buys them too. Not sure though
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"Captive, captive... captive in heaven" -Ertai, Captive of the Blinding Angel