This is a long shot, but let's see if anyone has any ideas.

Bought the car (06 wrx tr) about 6 months ago, in that time it has died electrically three times now (more below). The alternator was replaced under warranty the first time. The second time, it sat at the shop and they thought the alt. was dead again, then magically it came back. Don't know the verdict this time yet.

Mods: Cobb AP v1, xtreme xenon HID kit (grounded, checked that already), prosport electronic boost gauge, crappy panisonic cd player.... everything else is purely mechanical.

Symptoms: Slow or hesitant to start at times, it will idle ok, but then when I start driving, the power fluctuates rapidly to the dash lights, boost gauge, cd player, etc. to the point that they will turn off and back on again, and the dash lights flicker. The car even feels jerky, as if it's loosing ignition power. This goes away after 20-30 sec of driving.
Also, if I run the ac/defrost, when the ac cycles on, the dash lights flicker dramatically whether at idle or driving.

Most recently, the car has exhibited the above symptoms the last few days and then after driving about 20 min, turned it off for 5 min and then it wouldn't start again, no click, didn't try to turn over, nothing, but the dash lights came on. As with the previous 2 times it didn't start on its own, it jumped fine and started right up and ran fine.

The dealer insists the battery is fine (I've checked it myself and it seems ok), the alt is only 5 months old. Any ideas?

Thanks!

From what you say there may be a couple of issues with the electrical system or they may be tied together some way, possibly bad grounding between the battery and the chassis.

You may be able to track down the faulty power connection to the dash lights by tapping around the main power panel under the hood with a screwdriver handle. You could also try the panel in the dash area. There may also be a problem with the engine grounding so go over that also. One other thing to do is clean the battery connections using a battery cleaning brush. Even if the connections look ok. This will positively eliminate the connections as a trouble source.

that sounds like alternator diodes and or a loose connection

that sounds like alternator diodes and or a loose connection

I thought the first also, but that means either a.) the alternator died, came back to life and died again in 4 months, or b.) the replacement alt. was bad to begin with.

Anyway, thanks for the suggestions and keep them coming. It's a the shop right now since it's still under warranty, but I'm going to go over there today or tomorrow and talk to the tech.

sounds like a kid job on the electrical system bites somebody in the ass to me

i'd check for bad grounds and reverse all the electrical mods (HIDs, CD player, boost gauge) to be sure that they're not the problem.