imac dual g5 cooling fan probs?

Just got a g5 dual , third owner.. runs awhile then has minor video corruption and freezes.Playing video such as GarageBand gradually gets chaotic! Back of box gets a bit warm! Assume it is a cooling issue, everything inside seems fine, no bust capacitors etc. Fan seems to get quieter before failure...is this a common snag?
512 ram fitted, upgrading to 1gb soon, if this fixes it great...

Have you cleaned it to remove any dirt. dust, fuzz, etc.? The PM G5's move a lot of air…and they can get pretty dirty inside.

Pay special attention to the fans and the cooling fin areas of the cpu heatsinks.

- Nick