Phil,
This problem I had was quite extreme. The servo would move to and 'stick' at full travel, it would then be totally unresponsive to input from the Tx. If I nudged the control rod it would 'jump' to a new position but remain unresponsive to Tx input. After a few nudges it started working again. This fits perfectly with the symptoms i had when i crashed on first outing (uncontrollable pirouette) but as the servo was working normally after the crash at the time I didn't suspect a bad servo.
A slight 'fidgeting' of the tail in the hover is something different and is most likely due either to FBL setup or a little stickiness in the tail linkages. It's actually very common and if you are running low headspeed might be something that you just have to live with. I wouldn't see this on it's own as reason to change the servo, but you might check that the servo doesnt have too much 'slop' in it's gear train as this could be a cause too.
If you were in the market for a new servo for whatever reason the KST brand do offer great bang for buck and are very reliable. The 'basic' HV brushless BLS815 on cyclic and BLS805X on the tail work great at a price that's hard to beat. If budget is not a consideration then the options are wide but MKS has the reputation of being the Rolls Royce off servos.