I have gained some insight into the problem by looking at the files on the data card. I notice that there is normally one .001 file generated for each session at night (I normally have 3 or 4 sessions because I get up to get a drink when my mouth is dry). For the most recent occurance of this problem, I noted that there was an extra .001 file generated immediately after I turned off the machine after the first session. This extra .001 file has its byte number 16 set to 5. I've also noted that this kind of .001 file normally follows every night's worth of data. I know that if I shut off the machine for more than half an hour during the night, it always generates this kind of .001 file and the morning's display is likewise for a partial night. This most recent occurence, however, showed not a half hour, but less than one minute before the .001 file was written.
I'm wondering whether this behavior might somehow be related to the modem on my machine. The modem is supposed to upload the night's data every morning, and I've frequently noticed it doing that. However, I've also caught it on occasion uploading in the middle of the night (3 or 4 a.m.). I haven't been able to determine if there is any correlation between this behavior and the extra .001 files in the middle of the night.
Any comments or suggestions would be welcome.