If it randomly trips with the load-end of the SWA isolated then it confirm the IR results that the cable is damaged somewhere.I should also say that this swa cable has a 63a mcb at the front end and is randomly tripping ?
Most likely you see your 0.7 M ohm result after it has gone "Bang!" internally and you go to test it, then after time water creeps back in and it goes bang once more, etc.
There are techniques to help locate where along the cable a fault it, but some depend on it being a low and stable short (so you can see the relative resistance to a fault from the cable ends to guess approximately the ratio of cable to it), and others require expensive time-domain reflection sort of equipment (to look at the waves bouncing around just after it clears a fault). Has there been any civil works, digging of gardens, etc, recently? That might point to a likely location.
But it could just be poorly installed cable without sand/fine gravel around the cable resulting in a stone crushing it over time.