I downloaded some of the compatible stock ROMs from sammobile and tried them, but no matter what the baseband doesn't change, even though the build number and AP do change.
I'm flashing the stock ROM by doing this:
1-Turn off phone
2-Open Odin 3.07 and select the ROM (the .md5 file) on PDA.
3-Turn the phone on download mode by holding the Vol down+Home+Power buttons.
4-Attach the phone to the USB and let Odin recognize it.
5-Press start and let Odin do its thing.
I've already tried to flash the baseband separately by following this thread, but it also didn't work. I got an error message on my phone "secure check fail: ID 0xc" right before it reboots to complete the flashing. Odin gave me no error as if everything had worked.
My baseband is S7562ZCMB1 (is also the CP), my AP is S7562XXBMJ1, CSC is unknown (phone is rooted) and my build number is IMM76I.S7562XXBMJ1
I've also tried flashing the custom ROM with my baseband but, as expected, I had no network and had to go back to the stock ROM.
Please, help me! From the few minutes I tried PMP Ultra ROM I found it amazing, I just can't bear this sluggish phone anymore.