Can I make my iMac work better till I can replace it?

Imac 17" Intel dual Core processor

I crash several times a day while in Illustrator mostly. I get things like my curser sticks to something and won't let go - crash. that happens when I'm in firefox too.

I have a digital line through my sceen. it comes and goes. If I change the resolution down and then back up it goes away.

Last year my son installed Diablo 3 on my iMac. So it would work, I did a run-around so the graphics card would work with it. Since then, Diablo has been uninstalled. Any advice on how to go reset what I did for diablo? Is that maybe an issue? Before that I don't think I had any of these problems. In fact I remember thinking for such an old machine it was doing pretty good at that time.

I've been advised to take it apart and try cleaning out dust etc. I need a manual for this ;-)

Thank you so much in advance to anyone who can point me in the right direction.