At&T S3 on T-mobile. Unable to send text messages

[bbq9]

I'm using an AT&T S3 on T-mobile, with a custom ROM (i747z), and I keep having issues sending text messages. Sometimes the messages went through just fine, but some others, it keeps saying "sending", and I've been having this issue on/off for a while. Already talked to T-mobile but no help. Then I finally factory-reset my phone this morning, but same thing again (was able to send the first message, and the 2nd one got stuck right away)... Incoming messages seem to be fine.

I plan to try another ROM later on, but just want to check if anyone else has the same issue.