oh I got it now. I thought you were facing 500 (internal server error). If they stuck at 500, it could be any of hundreds reasons. Which MTA you are using? is it your own SMTP or purchased from a third party. If purchased one, then they might be breaking up SMTP connecting after receiving 500 packets and then when you resume, they accept slow feed. Else if its your own SMTP server, connection might be breaking up after 500 emails for any reason. Another issue could be cron that gets stuck after feeding a specific amount of packets to MTA. I am sure that campaign doesn't auto pause in interspire but it gets stuck, and you need to pause/resume it. It means the two possibilities. 1) connection is breaking with MTA, 2) cron stuck. Meanwhile when you send the campaign, connect your server over SSH and run -> "top" to find the average load of the server. If MySQL or CPU is overloading after the campaign starts then it means interspire is eating more resources than you have.
