Intel Mac :: Bad Crashing, Freezing, Lines And Pale Patches And More?

I wonder if anyone can help me. I am not very technobod and there are progressively awful problems on my Mac until it is now so slow.I need to fix it before it dies altogether, or I throw it out of the window.....Problems:- Thin black lines randomly appearing across the windows. Ghosting of windows, they leave another version when moved. Sometimes windows just white out in areas with scribbly lines like pretend text. Sometimes windows have large bare patches all of a sudden or go see through Programs keep just quitting for no reason. The whole computer just freezes every now and then. Sometimes it waits and returns, occasionally it crashes completely. It is really painfully slow and can take me considerable minutes to open a new window or get something else up on screen. The tool bar sometimes goes all skeletal with no background. Loads more silly things like this... all random. 

It is driving me absolutely potty. It's only used by me doing emails, filing pics, designing jam labels, letters, going on forums and things. I don't give it loads to manage unless I'm doing something spectacularly stupid. This computer ought to be big enough to run a design studio..I have tried mackeeper, found it gave more problems than it solved, and uninstalled that, I've tried Clean my mac and only cleaned some, I left the universal binaries and the Caches alone. I then tried Onyx and fixed some listings (I kept the report) and cleaned up the caches in that, then restarted. It is now even slower than before I started fixing it, and all of the problems are still here. I've got time machine for back up. What can be going wrong?

Info:
iMac, I mac 6.1 intel core 2 duo