Sometimes it is difficult to repro a problem at will, it would be better to have Fiddler running continously until the problem shows up.
For that reason, circular logging would be beneficial: we could set ,say, Fiddler to remember only the last 100000 events ; newer events will overwrite the older ones.
On Fiddler's toolbar, use the Keep Only dropdown to configure your limit.
Alternatively, use the View > Tabs > AutoSave mechanism.
Alternatively, use FiddlerCap (http://www.telerik.com/fiddler/fiddlercap).