Yes, the sear is slipping off the bolt. You're not alone, and other people have seen this problem with their guns.
I've had this problem too and it can be pretty scary and annoying to fix. The first thing I would do is remove the bolt and try removing any excess lube, especially around the middle of the bolt where the sear catches. If that didn't work, remove the grip frame and take a look at the sear itself. Make sure there isn't any buildup of lube on the sear that could be causing it to loose its grip on the bolt, and wipe it off. If the problem persists, remove the grip frame again and take out the little spring that holds up the sear. Carefully stretch this spring just a little bit, as that will make the sear hold onto the bolt more tightly. Be careful not to stretch it too much, as doing this will make the spring too strong and the gun will refuse to shoot because the solenoid won't have enough force to overcome the spring and let go of the bolt when the gun fires. On the UL SLG you can compensate for this somewhat by raising the dwell value, but the circuit board on the basic SLG doesn't have this feature.
Hope this helps.