Log analysis, random loss of control

Hi all,
I was flying my fairly new pentacopter (set up as a tri) around and about 30 seconds into the flight I temporarily lost r/c control. The copter jerked wildly back and to the right. In a panic, I quickly triggered the flight mode to switch from stabilized to land (switching through acro to get there) and it immediately righted itself. I opted to continue the flight, but I am afraid to fly it any more if this is going to happen again.
I am not very good at making sense of the logs, but the problem happened around line 1500 in the log (I think).
The pentacopter is running an apm 2.6 board with an external 3dr ublox GPS+Compass, and is using the 3dr power module. One of the esc’s has its power lead going into the apm to power the tail servo, but the other 4 do not. the receiver is a hitec optima 7.

Any help will be greatly appreciated!