Phone reboots while in G3 data

[gynos]

Whatever ROM i use, the phone reboots as soon as it connects to 3G net.
If i flag "Use only 2G networks", everything is fine (but with slow "E" connection).
This phone was originally Tchibo branded and i unlocked it via Unlock Solution
The same phone bought by Fonic works perfect with the same sim cards
Any clues?

thanks