Viber really has problems on our phones, with it even crashing on Stock android, so if you were to search around, you'd notice that most users recommend using an older version. I myself use 3.1.1
and it works more or less great. Just remember to disable auto-update.
"Many came and many left."
Device: LG Optimus 2X
Rom: Experimenting, but mostly Optix or TonyP's magic ROMs
LG GT540 Optimus
Apple iPhone 3G
LG Optimus 7
SE Xperia X10 Mini Pro