Old iMac : Screen Funkiness

I'm having troubles right now with my ( six year old ) iMac's display doing funky things. Every so often it'll decide to resize the display area to be taller and narrower, and every so often (rarely) it'll take that to extremes, start flicking around (kind of like it can't decide on a brightness level) and jumping around generally. When the former (rarely) happens, the iMac actually hangs (sound output ceases, no responsiveness, total lockdown).

I heard somewhere that this can be caused by a battery on the video card running low - this seems unlikely to me though. (why put a battery on the vidcard? what's on there that needs a battery?)

is this a Flat Panel IMac or the CRT IMac? What speed is the CPU?

Opps, slorry bout that, did not read your Rig Specs!

The older IMacs had problems with their monitors sometimes. Do you hear any snapping sounds when all this happens? It could just be the video board. I have one here but mine is the Flyback Transformer. Shorted out but before that it was making loud pops and the picture would blink and tear.

no snapping/tearing/shorting/arcing going on over here(as far as i can tell) - though when the colour of large areas of screen changes (like opening white windows from black backgrounds and so on) i hear a little (little incy-wincy) sound.

I had a 17" flat CRT monitor (recently deceased - CTX PR711F - **** nice monitor! ) that was doing snapping and chain-arcing sounds, so i know what that sounds and looks like ( sharp snaps, screen dies then slowly brightens up again - used to happen in the middle of Firearms 2.9c firefights all the time, dammit! ).

In any case, this iMac is just a stand-in for my Athlon 2000XP+ rig while it's getting fixed (again) - though admittedly i can do more on it than on my PC (specifically, Macs are very nice for doing web development - PHP, MySQL, Apache, etc. , though i haven't figured out how to build console C++ programs with XCode yet.)

Some old macs would not initialize the video if the PRAM battery was weak or dead but that was on models like the 6100. That is not an issue on the CRT iMacs (and there is no battery on the video board). It may be the horizontal control circuit on the CRT. Since the CRT is similar to a TV it can have problems that seem more related to a TV than a computer.

well i;m not sure what TV symptoms are, so i couldn't tell (until the CRT implodes, then explodes, inserting glass into various parts of my body at high velocities - a la the finnish guy from Cryptonomicon, but that's a different story)

I'm starting to think it might be stuff in there overheating (the computer's sitting in a corner, with no real airflow around it) - I've moved it out as far as my desk (crappy garage-sale furniture!) will allow, and i'll see what happens with it.