It's a mooseberg, so it's probably the safety. Or more specifically the safety button. The plastic moosberg safety button is notorious for cracking right where the screw goes through it to attach it to the slider underneath the top of the reciever. If that's the problem, then have a gunsmith install an aftermarket steel safety button, it'll fix it permanently.

wojtowicza,

Had the exact problem with a Mooseburg about 20yrs ago. It cost me the shot of a life time on a drive at a 12pnt buck at 20yrds. I was not a happy camper and will never forget that lost shot. It was the friggin cheap plastic safety as jp4 mentioned. Had it replaced by gunsmith. He installed steel version. I still have that gun and another Mooseburg 500. Both have taken about 40 deer.