SanJ,
While resuming a mailing I have not been able to get it to stall by loading the resume script. With debug info enabled it appears resume.php is working fine:
batid 630db0 found not completed.. checking if it's active.. YES - don't resume
I have also found your server to keep running for a time after closing the browser in the midst of resuming. This is normal - scripts can run for a short time or until completion when the browser is closed. Further calls to resume.php have the same result - no affect on the mailing. The resume script simply checks to see if the queue is completed or not and if it has not been active in the last minute will resume the mailout. This must not be the problem.
Considering you are attempting to achieve 500 emails per hour I highly recommend lowering the "Reconnect every X emails" SMTP setting from 5000 to as low as 100 or even 50. My reasoning is the server may not like having an SMTP connection open for an entire hour.
Please let me know if you continue to have troubles as further investigation may be warranted.
Regards