There is definitely malware - I suspect it's coming from one of your advertisers since it doesn't happen all the time. http://www.bondagezilla.com maybe?
http://http://www.avgthreatlabs.com/...smlibrary.com/
http://www.google.com/safebrowsing/d...smlibrary.com/
http://evuln.com/tools/malware-scanner/bdsmlibrary.com/