Don't think it can be anything to do with not using a goldcard, if that was an issue, downgrading wouldn't have worked.
Which rom have you installed? Did you wipe all partitions (apart from sd card)? Have you tried any different roms to see if that makes a difference? Are you using GSM or UMTS?
It's possible that upgrading your radio might solve the problem, as you will have an old radio (from the downgrade process), but be careful! If something goes wrong flashing the radio it could brick your phone.