[Skie]

Show him your BQM graphs (might need to print it COVID social distancing and all!) and explain the inconsistency of it.

Tell him what has been replaced and what hasn't, and any fiddling you've done (ie, if you've swapped attenuators, swapped the modem onto the V6 cable to rule out a dodgy cable between the splitter and modem etc.)

I had really similar problems that ultimately took an amplifier replacement to sort. It takes a lot of browbeating to get them to look that far up the chain (or them replacing everything up to the tap, as was my case). Intermittent faults like this only get spotted by some customers, and VM's monitoring doesn't notice it as well as it should.

Hopefully your issue is just a damaged cable in your home or garden, and the engineer can put a new one in easily.