iMac G5's are strange creatures.
My iMac G5, has six leaking caps, a messed up whining Power Supply. And what would you know, it works pretty well! It freezes and stuff, but I don't know if that has to do with the caps. It's slow sometimes. You probably would want to swap the Mobo, or swap the caps.

My iMac G5 had the same issue. I actuallyt had to get the logic board replaced twice within a 4 month period of the first failure. It could be that this board that you have now has also bit the bullet.

Mars478, the issues you're describing has to do with the capacitor problem. At this point, your best bet is to replace the caps yourself or have it done by someone that can.

A few years ago we had a few 1.6GHz 17-inch imac G5 systems at my high school, we took 'em to Vegas and had both of their boards replaced and the systems generally cleaned up. In a few weeks/months, one of them was back to generally misbehaving. A few months later I'd been told by the teacher to expect that the systems would be heading toward the proverbial "dumpster" of the school district. I haven't spoken with her very much since then.

Due to the nature of that program (Photoshop CS3/CS4 these days) and the way G5s are, the only PowerPC systems in active use are a powermac g3 that has a film scanner hooked to it, and the G5 at her desk.

We spent maybe $3700 if not more on that last-gen dualcore 2.3GHz G5, popped 6 or 7 gigs of ram into it, upgraded the graphics card to the nVidia 7800, and generally tricked it out. And a few months later, grabbed a 1.83GHz Core2 mac mini (GMA950) and were unsurprised but saddened to find out that it did most if not all of her desktop tasks better and faster than that G5 did. "But it's the server," I insisted.

And so it was.

My long-winded point is that iMac G5s have been for a long time, and continue to be odd creatures. Even if for whatever reason I got one that worked fine for free, I don't know how long I'd trust it to continue working fine.

Also, since you listed every single part in the machine except the motherboard, I'm going to go with Mars478 and Quad650 and go "It's probably time to either fix-again or replace-again that board."

Addition: Or the processor, I suppose the processors aren't socketed in any way though, so, "the board" would be the part you may want to look at replacing.

I just noticed that the power supply caps in my mom's iMac G5 are starting to bulge, so she will be getting the new power supply, and the rest of the sick iMac will be chopped up for parts.

Should an iMac G5 be able to play a 720p video? Mine can barely play 480... I think there's not enough current getting to the proc so it's stuttering. Time to replace caps.

I don't think you're going to have great luck with a single 1.6GHz G5. Try making sure your Energy Saver settings are set to "Better Performance" and give it a go. Also keep in mind that the codec used to encode the video makes a difference. H.264 takes more horsepower to decode than other codecs, for instance.

Changing caps probably won't help you at all.

Peace,
Drew

HD video is not going to come out well on a single processor system. It's just too much data to be shuffled around. I tried a USB HDTV tuner dongle on a single processor system and it was unwatchable. The instructions stated that a fast multi processor or multi core CPU was required.

I was playing 720p video successfully on the iMac G5 before it was chopped. It was a 1.8 GHz model.

You will want to use VLC, which is faster than QuickTime due to the ability to disable the block filter. Go into the preferences and disable the H.264 loop filter, then play your 720p QuickTime video which will run flawlessly (other than a slight loss of perceived quality due to the lack of block filter)