I have a 1187 which functions properly 99.5% of the time. It always functions correctly with no shells in gun. It will intermittently "hang or lock up" when the bolt is cycled manually and loaded. The bolt will retract 3/4" which is enough to see all of the brass and about 1/8" of the plastic part of the shell. The interrupter latch is out of the way, the locking block is retracted but the bolt will not go any further. This is intermittent and occurs randomly with regard to the number of shells in gun. Any ideas regarding the possible cause are welcome.