Government shutdown and Hillary releasing her book.
This, but Hillary's role in Whitewater/Travelgate was probably the bigger driver. Republicans were spreading rumors of her being indicted around this point.
The 42% seems something of a Gallup anomaly, as CBS/NYT and ABC/WAPO had Clinton in the low fifties that same week.
http://webapps.ropercenter.uconn.edu/CFIDE/roper/presidential/webroot/presidential_rating_detail.cfm?allRate=True&presidentName=Clinton#.TwihWaVSRvY