weird electrical problem....

I have a '98 XJ, 4.0, auto. with an odd electrical issue. Jeep will not crank, and at first I thought the battery was dead. Tried to jump it...nothing. So, when I turn the key, gauges perk up...then when I turn it more to start, gauges all die out. Also, when I turn the key, hit the brake pedal...gauges die. It's very intermittent that gauges perk up too, sometimes you have to try it a couple of times. A couple of times when gauges come up, without touching anything, the key-in ignition alarm sounds on and off rapidly as gauges flip on and off.
I don't even know where to start as I'm pretty much an electrical troubleshooting dummy on vehicles. I would very much appreciate a nudge in the right direction. Thank you.

First thing to do is to pull off the battery clamps and clean them and the battery poles. I use a 3M pad or you can buy a cleaning tool cheap at any auto parts store. Wipe down the poles and inside the clamps with a solvent being carefull not to wipe any oil or grease onto your shiny new surfaces.

Pry the clamps open a little with a large screw driver and avoid over tightening the clamps or they may crack.

Get a good charge on your battery. Pulse chargers are fairly cheap anymore and work well. A pulse charger can resurrect a marginal battery.

It may be grounds, loose power wire to the PDC, a marginal battery or something else, but cleaning and tightening the battery clamps is the place to start.

Thank you 8Mud.

It could be the ignition switch acting up. On my 89 it's located above the steering column by the peddles, a rod runs down from the key lock on the column.

Sounds like a faulty ignition switch to me. Has it always done this or is this something that started recently? A good place to start would be to check your ground from the battery to the chassis. Personally I think the factory grounds are weak and have always added a good 4awg ground from the battery. If you do this make sure you grind away the paint at the surface where you connect your ground and use a good sized bolt..

are the battery cables hot after you try a few times?
a weak battery as well as a bad cable connection will do exactly this. pull the terminals off and clean them, even if they "look ok" .
an old starter that is starting to draw more amps will amplify the problems a bad connection gives you.

Electrical problem? Driveability issue?

ALWAYS START WITH THE BATTERY.

Make sure it is fully charged. Load test with a carbon-pile tester, 1/2 CCA rating load for 15 seconds and the voltage should not drop below 10 volts.

If the battery tests Ok, make sure that both ends of both main cables are clean and tight.

Still no joy--perform a voltage drop test on the main cables. Google it.

Diagnostics is a PROCESS. Skip one part and you WILL end up chasing your tail at some point.

Thanks for the hints everybody....it ended up being a ground wire. The battery connections looked good on the outside but they were pretty bad on the inside(thanks sidriptide). It's a little embarassing I guess that I didn't spot it right away. What threw me was the inconsistant symptoms...thanks again, all!