If you remove the frame from your marker, you will find that there is a small lug that holds the trigger on the pin that goes through the frame, about which the trigger pivots. Make sure the lug is tight, and you shouldn't have any side to side movement.
Also, make sure there is no dust or debris in the area of the trigger beam. I had a piece of shell (how did that get there) that was causing unpredictable firing for a weekend.