Hello Stu,
I've had the same sort of thing for 6 years or so, since I began monitoring, and I'm always on the look-out for the answer. I've ruled out in-house causes by running router off a battery, monitoring via a laptop and by turning off the house main power switch; that does not affect the high CRCs. I think I've ruled out most neighbours premises by seeing the problem whilst their house is empty at holidays. Like yours, the high CRCs aren't matched by any change in SNRM behaviour, and there is no detectable drop of throughput until the CRCs get into many hundreds per minute (that is very rare).....and the typical rate, when it is happening is up to 60/min - like yours, very sporadically....sometimes absent for weeks at a time,.....sometimes once a day, but usually for hours or even days continuously. In the main, the problem is absent from late autumn to early spring.
It didn't use to bother me so much when on 20CN as it was very rare for DLM to take any resulting action, but now I'm recently on 21CN I find that its DLM is much more aggressive and is very likely to alter SNRM/interleaf depth etc......which then need correcting.
In recent times I've begun to form a link between the high CRCs and atmospheric disturbancies - thunderstorms - and by no means only local ones - storms 100+ miles away can often be correlated......I'm assuming that my 5km line with its high attenuation/vulnerability (69db for much of that time) makes things worse. Local thunderstorms, say within earshot, do affect SNRM too,,..and can (infrequently)make the CRCs rise into 1000's/min - answer: turn off router.
Jack
