I'm sure it's too late for the OP but after trying everything to fix this problem & endlessly searching the net I eventually figured out it was actually my old, out of date SIM card.
It's something to do with the card being not fully 3G or early 3G. Anyway once I got my provider (Vodafone IE) to give me a shiny, new SIM - problem solved.
I had been running fine on CM7 it was only when I flashed the ROM to Ginger Stir Fry (& Blue Ginger too) that it started.
Hope this helps. Let me know on Twitter if it does
@emcg53