Uninstall the NTFS software, then reinstall it.
From what I read, you may be SOL with the free version on Lion, perhaps because of no 64-bit support. You went past that when you upgraded to Lion, which defaults to 64-bit.
If it does work, and you want 'free', then you may have to put up with some instability, and certainly slow performance.
If you want that NTFS drive to work "OK", then you need to get the commercial version.
My feeling is that you want to ask yourself if you really need to share your drive with a Windows PC.