phone lose data whan connecting 3G networks

ok this problem is really irritating lately i'm facing the issue that when the phone switch to 3g network i lose all connectivity when i switch to only 2g everything is alright but the second i switch to 2G/3G MOD phone loses all connection
i have tried number of roms and radios. still same issue..

anyone can help?
im corrently on candyrom 3.1 (5.1.1)
and with ucnj1 modem