2000 sport electrical problems

So my 2000 Jeep Cherokee is hitting 100k and things are breaking. The latest fatality seems to be the electrical system. Funds are low from the previous repairs though so I can't afford to take it to the mechanic again just yet. Here's the symptom:

Yesterday while idling the lights and radio cut out for a fraction of a second, then continued. I got home and turned it off, and immediately there was no power whatsoever. No doorlock power, no lights on the dash when the key is turned, no interior lights. A few hours later I go back outside and there's an ever so faint trickle of power lighting the interior lights when I open the door, so I put the battery on the charger and let charge overnight.

Next day I crank it and it starts beautifully. I make a trip down the block, and return, and turn it off. I wait a few seconds, I turn the key to the ACC position and the dash lights and the radio turns on. I turn it further to start it and all power dies again.

I connect jumpers to the battery to test for power and there's no sparks when I clink them together. I assume battery is dead again, connect the battery charger to it again and power it up. While fixing the clamps to the cables at first nothing happens. I re-adjust them and the lights suddenly turned on. And the engine now starts again.

Anyone got any ideas? Probably not the fuses since it's cranking again. Battery is about 4 months old, recently replaced. Possibly defective? The battery connections are tight since I disconnected them when I replaced the spark plugs a couple weeks ago. Thanks!

Sounds like a connectivity problem with the battery. First, have your battery tested to see if it's good. (Many auto parts stores have a tester)

Next, be sure that your cables are in good shape. I've had two vehicles where the inside of the cable was corroded - which in one case caused the car to die, the other all sorts of funky electrical things happened. If that's the case, replace the cable (It's usually the positive side)

Lastly, make sure that the connections - both at the battery and at other attachment points - are good. A loose connection seems possible since you said that when you fiddled with it the lights came back on.

Check the battery again, with voltmeter (should be 12.6 volts), then the postive cable and ground cable.
Do you know how to do a voltage drop test? to see loss in the cable?
Do know how to do a load test? Then a battery drain test (with ammeter)?

Load test should be above 9.6 volts (measured while still cranking) after 15 seconds of cranking with all the headlights and such on and the distributor coil disconnected and grounded. (Get a helper.)

By the way, I replaced both positive and ground cable for corrosion, they are cheap, a little over twenty bucks.

Then, I would go to the ignition switch, because that is what happened to my 97 Jeep when it had about 90K on it. A long story, but the dealer had a hard time with it also. I finally went down there (to L.A., SoCal) and had the foreman work on it. After we took it back four times, a battery, and a starter they figured it out.