[Flatlander]

I had the same problem with the same gun. What I did was pry the 'finger contacts' up slightly so they make better contact with the selector plate contact.

These finger contacts are two little prongs located near the front of the selector plate between the plate and the mechbox. As you slide the selector plate over these prongs, they make contact with the selector plate contact (strip of brass) and it completes the circuit. Over time/use these prongs get pushed down and don't always make proper contact in the semi-auto position and eventually the full auto.

Extremely easy fix. Just pop the lower with the mechbox out and I'm pretty sure you have access to pry those up slightly.