My Thunder .380 fires double action only (DAO). This problem comes and goes. While it is active, the hammer cannot be latched by cocking the hammer by thumb, racking the slide, or by firing the pistol. A gunsmith repaired this a few years ago, but now the problem has returned. The pistol is kept clean and lightly oiled, has fired perhaps 500 rds., and is in perfect shape. The problem occurs with any type of ammunition. Sometimes while I am shooting, the problem goes away. Any advice for me?