That happened to my friend all the time right after he got Frontiers. He got so fed up he just went back to his original disk. Which wasnt all that big of a deal since at the time his main was a lv30 shaman. After a couple of months when he got into the higher levels and was having problems finding grps since he couldnt goto Odus, he decided to try it again. When he went back it worked like a dream. Basicly this doesnt really help you to much but, just letting you know your not the only one.
When this happened had you been playing for a long time? Sometimes if I play long hours mine will freeze and when I try to reset I get a disk read error. When this happens I take the disk out and its really warm. I usually then just switch to my other PS2, hehe. This has only happened to me 3 or 4 times though.