My 1999 XJ8 would lose all power to the dash,lights,starter everything. When I manualy open the boot,and check the battiery connections,the power comes on ! I have checked the connections for signs of trouble,but cannot find any,fuses,lose connections,grounds.Oddly enought,one time there was power at the 200amp fuses and the connections after but not at the engine compartment terminal

 

There's a common failure point in the wiring loom on the rhs boot hinge, you'll have to strip back the loom tape and inspect each wire for breaks - as you get power when opening the boot, you may have a short in that area....

 

I had a similar problem. No power. Open the boot and wiggle the ground strap and power comes back. For a while I carried a 10mm wrench in the cubby. I replaced the graound strap with an old-fashioned solid cable and cleaned up all the ground points. No problem since. But, I also had the boot hinge wire break problem. That is another story.

 

That makes sense the ground wires on my power ant gave my trouble,I had to replace them.Thanks again! I'll change the ground shunt with a standard ground cable and see if that does the trick