I've.akso been having severe speed instability issues...in my case, related to a fried power supply, just as @effischer describes above (same deal, too, fried in transit, loose transformer,. Etc. Etc). I've temporarily replaced with a $10 24v wallwart power supply I had lying around, but predictably, it sounds horrible with wow and flutter. I was wondering, @lewm , wouldn't something like this function quite well (may even be overkill)? :
http://www.ebay.co.uk/itm/50VA-Hifi-linear-power-supply-LPS-DC5V-9V-12V-15V-18V-24V-for-choose-ZJ-91...
http://www.ebay.co.uk/itm/50VA-Hifi-linear-power-supply-LPS-DC5V-9V-12V-15V-18V-24V-for-choose-ZJ-91...