Sound like you may have old immobilizer somewhere cutting power or perhaps the ignition barrel is faulty. As well as checking the earth also check the positive from the battery to the starter and then from starter to the alternator. I once saw a W reg xr3 with a weird no power problem and it turned out the battery + lead has a fuseable link inline with the cable that had blown. You can get a cheap multimeter for around a fiver and I am sure it will pay for itself in no time.

Remove steering column shroud and check for burnt out wiring 1st - seen this before and its an easy/quick thing to try first.

Good luck - post up how you get on