Problem with my playstation 2?

ive had my ps2 for 6 years and it was used when i got it so its older than that, just recently it hasnt been playing dvds right. some dvds it will play fine and others(even brand new dvds) it wont play at all, constantly freezing. its been a great system with this being the only problem in the 6 years ive had it, is there anything i can do to fix this