I'm havin a pretty similar problem. Back in July, I was playin Zelda the Wind Waker and all of the sudden the screen jumped and all of the torchlights could be seen through the walls...It happens anytime you play the game for an extended period now.
Then in september, I rented Splinter Cell and my sister got Pikman. Splinter Cell worked fine for me, but Pikman would freeze after you got past a certain part in the game.
Move on to October, I rent F-Zero GX and Soul Calibur 2. SC2 works great, no freezes or anything, but F-Zero GX is all screwed up. Sometimes itll just randomly restart, sometimes the graphics will get all jagged and distorted, sometimes itll just lock up. So i exchange it for another disk of the same game, and the exact same thing happens.
Up to November now, I just rented Rebel Strike and i pop in the disk. I watch all the movies and start out on the first level. After two minutes it freezes. Then I move on to the coop, the first two levels work fine, but once we got to Hoth, it would freeze (sometimes during cutscenes, a couple of times during the first part, but mostly during the wrap up atats part). The Versus mode works fine (of the ones weve tried) except that all of the Geonosis levels freeze right before they finish loading. I exchanged the disk like i did with FZ-GX and the EXACT same stuff is happening with the new disk...
Wierd thing is, all our old games (Zelda:WW, Metroid Prime, Bounty Hunter, SSBM, Mario Sunshine) work fine, except for the problem i mentioned in Zelda and we did have the Metroid Prime elevator glitch and some of the new games we rent (Splinter Cell, Soul Calibur 2) work fine too.
So, do you think my GCs about dead too? If so, why do some of the games work while some of em don't? It also seems in Rebel Strike that if you play the game after you havn't had the gamecube on for a while, it lasts longer, like it crashes because it gets too hot or somethin. Its also alot more likly to crash when theres a lot of action on the screen. But anyway, what do you think? Think the GCs about dead and if so, why aren't all the games having the problem?