Problems with my remington 870?

i will try to explain this as best as i can. i've always known these things to be dependable workhorses, however, i have been having a bit of trouble with mine. occasionally, the forearm will slip back, opening the chamber, as if it is not quite latching into place on the upstroke. sometimes when this happens, it will actually jam with the chamber half open. i have actually tried to get it to do this on command and have found that if i wiggle the forearm when it is all the way forward, it will slip back. no idea what might be causing this.