phplist sends out messages via cron but isnt consistent

Hello, i setup a phplist system with about 4000 e-mails. I send out a newsletter daily using rss feeds and cron jobs to do the sending. This whole process is automated, with the cron getting rss, the message being created daily by embargo(or w.e) and then the message getting sent out an hour later.
But latelly iv noticed that phplist decides if it sends the full ammount or not. Like some days it sends out like 500 e-mails only, and other days it sends almost the full 4000 e-mails.
Does anyone know why phplist is so inconsistent?
I know it isnt my cron jobs, as the cron job for sending e-mails runs hourlly, so every hour after it finishes the 450 batch, the cron job for process queue runs again and dose another 450 batch.
Can someone help me out...
Thanks, Dani
But latelly iv noticed that phplist decides if it sends the full ammount or not. Like some days it sends out like 500 e-mails only, and other days it sends almost the full 4000 e-mails.
Does anyone know why phplist is so inconsistent?
I know it isnt my cron jobs, as the cron job for sending e-mails runs hourlly, so every hour after it finishes the 450 batch, the cron job for process queue runs again and dose another 450 batch.
Can someone help me out...
Thanks, Dani