This phone appears to disable the JTAG interface after finishing bootup.
If you can't get the phone to halt over JTAG fast enough, consider enabling USB download mode as instructed below.

Unfortunately, the keypad combo to activate this is not yet known, so you'll have to switch the phone to USB Download Mode in software.

To switch to USB Download Mode in QPST:
- Connect the phone to your PC and select the SAMSUNG diagnostic COM port in QPST
- Open the MemoryDebugApp program (should be in the /bin/ folder where QPST is located)
- Click "Get Regions" and the phone will switch to USB Download Mode