Sounds as if the tension is a little weak on the saftey/selector. You can give it a bit more tension to stay in the detent dimples on the reciever. Here is how: Remove the selector from the reciever- To remove just rotate the lever upwards until it lines up with the notch in the hole in the reciever and pull it out- Watch the retainer "Shepards Hook" for the trigger and hammer pins. With the selector out of the reciever just give the lever a slight inward bend. What this will do is once it is back in the reciever is put a bit more pressure on the dimples on the reciever and it wont walk out of them.
Notice I said SLIGHT bend. It takes very little to increase the tension. We are talking just a few thousandths of an inch here. Put too much and it will take too much to move the selector.
When I tweak them I just use hand pressure only by applying pressure to the far ends of each leg of the selector.
Reassemble and dont forget the "Shepards Hook"!

It took me longer to type this than it will for you to do it!