Only 2 emails are sent before cron times-out

I am trying to use SMTP module to send notifications to my 14 users through gmail. The test email works fine but, when sending the 14 newsletters (in fact, notifications from "notify" module), I get the "Cron run exceeded the time limit and was aborted." after just 2 emails.
The strangest thing in this issue is that if I turn on the debug option and call cron.php on the browser, I see that emails are sent very slowly (around 3 per minute) but cron actually completes and all emails are sent. All other configuration tested (debug on + crontab, debug off + crontab or browser) fail with the above message on the logs.

Can anyone provide some clue on what might be happening?
Thanks,

Pedro Miguel