I have gone through two PS2s because of this problem.
In the last two years, Sony has made eight hardware revisions to the PS2. Why? Sony used an extremely cheap laser for the DVD reader. When that laser goes, any DVD movies or games will not play. The part is currently on six month back order. I have been told that if I were to send my system in for repair, it would take them well into the next year to get the part and actually fix it. They sent me a new system, which now has the same problem.
BTW - there are three companies I know of that, as their sole business, repair PlayStation 2s. Each one of them that I contacted is also backed up several months.