Now then, this could be interesting. On a works DG this same symptom has been mentioned as a few EU's have experienced problems like yours when (and only when) they have been migrated from 20CN to 21CN. The EU's have also had numerous visits with no long-term results.
Apparently, this issue is now being dealt with at a higher level.
<Disclaimer> Obviously, each EU's 'fault' situation should be treated individually and on their own merits, as the above may only pertain to particular SP's equipment.