The error occurs long before it gets to the stage of deciding to generate the graphs, it seems to drop out without logging any modem stats as well, if you look in the log it states: 18/09/2017 06:01:02.459 - time_difference = 2 minutes
18/09/2017 06:01:02.459 - WARNING!!! time_difference is more than 1 minute - Ignoring previous logged time which implies it didn't record any stats for the last minute.
There is an option to allow for the speed of the PC, might be worth changing that, it's on the same tab as the error logging options.
Only other thing I can think of at the moment is to delete all error log files, turn on 'Enable extra debugging info logging', then once the errors occurred zip up the entire log files and attach them. There might be something in one of the other logs which will tell me what it is. I could do with a couple of hours worth of data - I want to see if it is regularly having errors, rather than just the time the graphs should be produced.