Mass Mailer Randomly Fails - Joomla! Forum - community, help and support

hey guys, our mass mailer has begun exhibit strange behaviour (after working fine quite time) sending emails out people not others. emails not turning in spam folders, aren't getting recipient , our site owner has suspicion that, since mass mailer interface no longer takes long refresh upon submitting message mailing, script failing before able complete of emails. suggestions? can provide further information if needed

welcome joomla forum!

what joomla version use? latest version 1.0.x series?
what php version use?