So, after about 15 months, I noticed our MX8 doing counterclockwise circles in the pool. It seemed that the drive on the left hand side of the unit had some issues. After reviewing the following videos on Youtube, I was able to fix this:
http://www.youtube.com/watch?v=jR_8eapr ... ature=plcp
http://www.youtube.com/watch?v=lobCDLH3 ... ature=plcp
http://www.youtube.com/watch?v=1UejteXA ... ature=plcp
These were posted by Zodiac and are very helpful in explaining how to service the MX8. It turns out the the directional control on the left side was getting stuck. I took apart the unit as described in the videos, and removed the left directional control. I disassembled the control and found that one of the two bearings that controls the movement of one of the bevel gears had gotten crudded up with what I think are calcium deposits. So, I simply brushed this out on the inside until the bearing turned freely on its axle. I reassembled the control without too much trouble (you do need to turn the gears until the directional control is in the down position), and then put the unit back together. Now it works just fine.
The point is that this unit appears to be very serviceable. Parts are available and reasonably cheap. So, if you find your MX8 is not responding properly after a while, try to service it yourself. The videos are a great help!