What do do with an old iMac that shuts down unexpectedly?

Our 6yo G5 rev B iMac is shutting down unexpectedly. It's mostly likely the power supply, which is sold for about $150 with shipping. (This was an exotic Apple model -- it was designed for easy repair.)

It's an old machine with a partially delaminating monitor. So it probably only has a couple of years left in it anyway. That argues for replacing it with a mini and a monitor for about $800.

On the other hand, the kids use it when their MacBook is busy and it doubles as a movie theater and media server. It can run Classic, so it's valuable to people and businesses running legacy software; it still has a resale value of about $450. Throwing it out will cost money, plus that kind of waste hurts my soul.

Yet again, do I know the sudden shutdowns are a PSU? Could be something else. The costs of investigation are high ... but not as high as the pain of buying and configuring a new machine.

Decisions ...

I'm probably going to do a hardware test (if it will run long enough), reset the SMC/PMU and physically inspect the capacitors and mb. If it does well there I'll order the PSU.

Update: Capacitors look good, but it shutdown during the hardware test. I'm 90% sure it's the power supply, there was a brief recall five years ago but it was fine then. It's long past the time even a generous vendor would replace a PSU, so I ordered a "new" one from iFixit. The quotes because "new" in this context means Apple sourced -- I very much doubt anyone is making truly new G5 iMac PSUs. iFixit was $15 more than the lowest price, but they have by far the best reputation of the vendors I found. I will also be using their install directions, so I like to encourage them.

Update 8/10/11: The replacement came from iFixIt in a couple of days. It was very well packed. iFixIt includes a mailer to return the old supply so it can be "recycled". I think that means returned to Apple to be refurbished. Please do that, there can't be too many of these in circulation.

As usual, installation was trickier than the iFixIt  video suggests. I suggest look at user notes on all the G5 supplies they sell to get more tips. Here's what I wrote:

1. It is very hard to disconnect the optical sensor cable from the motherboard. There's no hidden catch, you just have to insert your spunger/thin screwdriver blade between the fittings and gently twist. I used a surgical clamp to also pull on the cable -- that did it. (The cable is being replaced, so you don't need to worry about it).
2. The top right screw is hard to remove. I had to partly remove it, then lever the power supply up as in the video while continuing to loosen it.
3. I used thin wide screwdriver blades to disconnect the power supply connect, my spudger wasn't rigid enough.
4. The supply has clearly been used before. In the context of something this old "new" means Apple refurbished. That's fine.
5. Beware the optical sensor, it's glued to the power supply. Look for it. When you insert the new supply place it first.

Even though my supply lasted a reasonable (but not great) amount of time, I've been annoyed at Apple for the high failure rate on the G5 iMacs. Going through the removal process though, there's nothing "cheap" about this bizarre and exotic "power supply" with its attached optical sensor. Apple must have been very unhappy with the manufacturer; I bet there's an interesting story buried in this supplies high failure rate.

iFixit included a $5 off coupon code. I'll never remember it, so feel free to try it: 5611FIXIT.

Lastly, the repair does seem to have worked. With the old supply machine shutdown less than two minutes into the hardware test. This time it completed a 1.5 hour hardware test.