Ok, first question: what radio are you using? In principle any radio with _5. in the number should be ok, so it is odd if one is incompatible with 3 roms (all GB though). So I would try your old ROM first, as that is risk free. But a radio may be the answer.
Flashing radios is ok if approached carefully. First thing is to check the download is ok, i.e. check the md5. Some of the radios in the dropbox thread have md5 codes supplied, and I think these are now available for radios from the xda radio thread. Personally I'd check after copied to your SD card (AFV or ManD5 from the market).
Then you flash like a normal zip. It flashes quickly, then installs when you reboot. You must not interrupt this! So don't do this with a low battery, and don't pull the battery if you think something is wrong. It will reboot by itself a couple of times, so this is normal and does not mean you are in a bootloop.
That's all there us to it really. I've done this many times, but it shouldn't be done on a whim as a bad radio flash is likely to be fatal for the device. As always it's at your own risk. That's why I'd be tempted to spend a few mins checking the original ROM (or even a basic froyo) to check it is software first. But assuming it is, this is the best bet at a fix.
