Can anyone explain what might be causing my internet service to drop out with the corresponding message in the router log 'LCP is down' but my line is remaining stable with the modem synced and no noise spikes shown in RouterStats.?
I stand to be shot down by others.
LCP tends to be an authentication ISP issue and happens after exchange. Thus sync is not affected.
It is well documentation that many users get problems after 21cn exchange upgrades. Normally capacity is gradually increased over a couple of months, and is handled by OR.
Suggest you wait a couple of months and maybe ask ISP to check IP.
If still problems ask ISP if OR can reseat at exchange.
Maybe worth having SNR upped now to 9db temporarily.