The likelihood here is that your data source is updating less frequently than your Feed update rate (or the timing of the update is not very accurate). Generally it is better to have the Feed update rate to be longer than the data update rate.
[edit]
No idea
I’d run a tail
command combined with a grep
against the log file and just extract the log for one feed and see what happens. You could do this against both log files in separate SSH windows and then spot the events happening.