large mbox times out

I have a large mbox file - 700mb or so - and I've broken it down in to 50mb chunks just to be reasonable. When i query the mbox "test connection" it returns the correct number of emails - something around 5000 per 50mb chunk. when i go to import it rarely gathers more than 1000 at a time before going dead..... I've got batch import stuff turned on... just can't get any love out of this... thanks for any help

btw max execution time is 600, memory 1248