I recently obtained an Iver Johnson Model 55, 8 shot .22 revolver. I just used it the other day with no problems. After cleaning it this past time, I noticed that there is now a catching stuttering feel as the drum rotates at half cock. After looking at it for awhile and playing around with things, it seems that the drum is catching on the slight protrusion of the hammer at half cocked. It hasn't done it before and I don't think its supposed to. It just doesn't feel right and I'm worried the catching is going to ware down the firing pin. Any ideas on how to fix it?