I have found your problem

After convincing Cass to travel with you for the quest "Birds of a Feather", many of the weapons on the tables may be in very high condition (example: Multiplas rifle at full condition). However, taking the multiplas rifle (or any other gun as far as I know) will cause any attempt at lock-picking, hacking a computer, or looking through a scope to become a blank computer screen obscuring your view. You can still function however, but attempting to do any of the three things will obviously be extremely difficult. The only solution found so far is to reload to your last save before you took the weapon(s) and make sure they are not in a "to good to be true" condition when and if you try and take them again.
http://fallout.wikia.com/wiki/Silver_Rush