Regarding your electric fence:
1) You may not have a good enough ground. Check that the wire connecting it to charger is very tightly and cleanly connected both to charger and to ground rod; get out hose and water the heck out of the area around the ground rod for a while (this can really help sometimes); and it is possible you need more or deeper ground rods, although I'd be a little surprised if it were
that for such a small installation.
2) Turn off fence. Disconnect charger from fence wire. Turn fence back on. Put tester on the charger terminal that fence wire was connected to (w/other end of tester on the ground of course). If tester shows no charge, your problem is almost certainly with your ground... see above. If you DO show a charge, your problem may still be a weak ground (see above) or it may be a problem with your fence wiring.
3) Commonest problems with fence wiring: loose or grimy connection to charger terminal, or fence wire accidentally being touched by ANYthing that isn't a plastic insulator, or wire inadvertantly installed on one or more insulator in such a way that it runs very close to the nail and arcs a short. If these are ceramic insulators, it is possible one is cracked. If they are the 4" plastic insulators that use long nails, there can be problems shorting to the nail just b/c of design (4" ones w/short nails don't have this problem). And if they are the stubby 1" plastic insulators, there can be problems with wire arcing to other materials (like the metal pen mesh).
Another theoretical possibility is that your metal pen mesh is creating too much resistance for your charger. I have never seen this happen but I know it can in some circumstances. Solution would be to set electric wires out a couple feet from pen (recommend t-post corners with fiberglas line push-in posts from farm supply store, use gloves when handling fiberglas line posts)
Swearing helps too. i expect you've already tried that, but keep at it
Very best of luck with your Evil Predator. FWIW it does sound like a weasel to me.
Pat