edit: I tried a 60MB full res sample from internet and it was fine on my mbp3,1.
Does your imac video card support H264 codec natively? what OS X version?
imac6,1 is 2006 generation so there may be issues. Try getting full 4GB RAM set, it does help generally to make things faster. And update the software as much as you can, snow leopard would make sense for just £25 if you haven't got it already. Its the last OS it will run.