I have 2 66, both sound fitted locos. The colas one has ran fine from day one, the only issues being the axle box on one of the centre wheels etc on one bogie keeps throwing out as soon as the loco moves, it doesn't fall off, just hangs out by about 3mm. It can be pushed back but soon pushes out again. It seems the centre wheels et has more play in it from side to side, which causes the axle box to throw out. If I move it wheels etc side to side it does just that.
On my other 66, same issue, but I have a wobble mainly at one end. On the dcc concepts rolling road I can see that one of the leading wheelsets seems not to be fully in contact with the rollers and furthermore it also runs out of true, this seems to cause the wobble.
Now one strange thing that happens is that when running on the RR in one direction only at a moderate speed, the motor will suddenly slow down for around a second, then resume normal speed. When it does this, a high pitched noise comes from the motor. It does this around every 2 to 3 minutes of running, only one direction though, the faster I run it the more often it will suddenly slow down. It run perfectly fine in opposite direction and the sound is unaffected when it happens. I've also noted that at low speed steps, between 1 and 5..It is very jerky in the direction that has the above issue, in the opposite direction it's fine. I use an NCE power cab control system and have tried it on speed steps 28 and 128. In normal operation, to get the loco moving I wind the controller upto around 6, this allows a pretty gentle start. In the hattons manual it says not to alter any cv settings as they could effect or prevent programmed cv setting from working correctly.