the stock wires are designed only for 55/60watts and most Xenon bulbs are 90/100 watts so the wires will fail sooner or later. you'll have to keep pushing on the connections to get them to come back on but it will keep happening untill you replace the wire harness. Southwest Autoworks carries the upgraded harnesses for like 10 bucks. I replaced mine after having that pesky problem and haven't had a problem since.
here's a link.
http://store.yahoo.com/southwestauto.../wiruphar.html