Unlocked sim and upgraded using Itunes :(

I am 2G user and have possibility to to buy 3G for a good prize. The phone is from the summer 2008 but a seller has upgraded it using itunes on PC. I think it has now the newest firmware and baseband (02.30)

He told me he used yellowsnow on PC. He has also removed a simlock on PC (maybe he did it before upgrading from itunes, I guess now it is not possible)
He says it works in ERA PL without problems but while inserting another sim it losses the signal sometimes. I am not sure what he did.

IS THERE A POSSIBILITY to make it usable and remove simlock. I think the simlock has been removed before upgrading and now there is something like network problems on another network than ERA PL. If no do you think it will be possible soon? Is it possible to do a beseband downgrade or there is something wrong with the newest firmware?