Eric,
After a few days and more tests I can't get the past days traffic saved in file with either a PC shutdown or a modem power down. However only a PC shutdown does not seem to matter as the full modem values pick up on PC startup and with only a modem shutdown the program seems clever enough to cope and not go to zero. It seems to be the combined PC and modem shutdown that is the issue.
Might the save action which occurs on program exit be allowed to occur with each sample. Perhaps via an option if it is a significant overhead. I guess that would be one fix.