I have two 396xt's that work perfectly on P25. I also have a Pro-106 and had the Pro-96 before that.
My advice for the 396xt:
1. Leave the digital adjustments at default values. Unlike the 396T, the XT's auto default setting is best.
2. Do not update the firmware to the latest version 1.11. 01
Revert back to version 1.10.1 where necessary.