Im experiencing a problem nearly identical to yours. I have an i317 unlocked using a Tmobile SIM. Everything was working fine until I flashed a rom (I believe I accidentally flashed a rom with a Tmobile radio which boot looped the phone). After fixing the bootloop and flashing the cyanogen mod rom, I started experiencing the frequently dropped signal.
I have downgraded and gone back to stock several times which has not fixed the problem
After reading this thread I changed the factory mode script to off, but it doesn't seem to have affected the radio performance.
I remember reading that the partition could possibly be corrupted in a bad flash. What is the best way to reformat the partion?
Any help greatly appreciated