Might this have any bearing on what is happening to you?
Worse still, Nikon made the decision to make CFP always active if you select Dynamic AF and inactive if you select Single Area AF. Yes, you read that correctly: the F100 will not necessarily use the focus area you select if you set the camera's AF area switch to [ ], but it will if you switch it to [+]. Fortunately, you can override that decision with custom settings, but I just know it'll catch a few F100 owners by surprise someday.
I just tried changing the AF area, and now it works regardless of the mode selected! Strange. Maybe something is wrong with the thumb wheel. I noticed that it seems to happen when I am outside in very cold weather.
-20 C? I wish it were that warm when I've shot sometimes. I haven't used my F100 in uber cold yet, but both my Nikon Fm2 and my Pentax Spotmatic F work fine in -10F. The only problem with the spottie is that my lenses started to freeze up and not turn to focus.