Hey all!
I'm usually pretty good at figuring out problems like this, but this one's got me baffled. I'm an experienced pilot with lots of build experience.

It happens only about once in every 3-4+ flights, but every once in a while, the tail will all of a sudden yaw at a moderate, but not too fast rate. It turns I think (from memory) nose-left and I correct with right rudder to fight it. The yaw'ing lasts for about a second, then goes away and may not rear it's ugly head the rest of the flying session, or may do it the next flight. I'm running a 450XL CDE with a phoenix 35 ESC, JR PPM receiver, GY401, HS-50 tail servo.

Any ideas?

Sounds like the tail slider may be binding. Did you undo the tail servo horn and see if it moves freely?

The other thing may be a glitchy tail servo...I'd replace that one next if it's had alot of use. Maybe find a buddy you can swap parts with.