Please note that at this stage we don't officially support custom ROMs with Viber.
And no one should expect that. If I cannot exit and keep it closed, I can leave it on the background. My current ROM seems to have gotten me another 2 hours in battery life or so, an Viber eats very little. Everything else works.
* By the way, have you tried testing Viber on a Stock ROM on that same device?
Have been using Viber for over a year or so. Not a heavy user since most my contacts are not online 24/7. No reflection on the app. Have made 3G and WiFi calls on it. Embarrassingly on a good HSDPA signal or decent WiFi call quality is better than my GSM network.
Have sent and received messages. If off, it receives the push notifications and powers on after it gets the message.
But I cannot answer your question. Two (or was it three days ago), I downgraded my ROM, rooted my phone, installed a custom ROM and flashed a radio. The Viber update came after I did all that.
Viber was working perfectly with my last stock ROM. Never rooted or used a custom ROM before three days ago.