Did you get any further with this?
@eta's idea of doing the meter check is useful. It will give you an idea if the cable is broken or shorted somewhere along its length. Do the continuity test with one end shorted to look for a break in the cable. Then repeat the same test but with both ends of the cable open circuit. This will tell you if there's a short somewhere.
Do a double check on your plug ends, even if they look okay from the outside. It's rare, but I have had it where the outer sheath has stretched which results in the centre core and dielectric getting pulled away from the centre pin connection. Usually it's with cheap RG6 - the sort with the aluminium braid wire and copper coated steel core (CCS) - they use inferior grade PVC or it isn't bonded tightly-enough to the braid and dielectric core.
Some would say that soldering the centre pin would solve this, and they'd be correct. But the fact that an installer or home DIYer is using cheap coax in the first place kind of tells you where their priorities lie; they're not going to take the time and trouble to do this when the rest of the job screams cheap/quick. It's a basic contradiction of intent.
According to the info you posted from the screen shot you're on the Gloucester
Lark Stoke transmitter. Is that correct? If so, then there are 6 muxes all transmitting at around 2,500W I think. The point here is that there's no variation on power between the muxes, and so any big variation in reception strength is more likely to be down to the aerial and distribution system and its cables and connections.
