Yes, a 25% drop in the max attainable speed after 3-4 years of stability suggests a performance issue. Ten days of high latency as detected by speed tests (+150ms+) and now packet loss suggests that there is an issue. I appreciate that most people would regard 67D as a good FTTC connection but from where I am sitting I can see the guys working on the 2 PCPs and underground cable.
Until now, I have rarely looked at the BQM - my line just worked. The fact that the deterioration occurred after a midnight event suggests, to me, that it is maintenance related. The ongoing PCP change is a factor that isn't helping. In fairness to the guys, they are trying to sort out a mess not of their making.