I had a problem like this. I traced it to the fact I was running that particualar game off the hdd. Which doesn't seem to be your problem. Well, before you do anything, clean off the optical heads on you optical cable (with a qtip and alcohol). If that doesn't work, take it to another receiver and see if the problem is the same - or use another optical cable. I have trouble believing that the optical out just f'ed up all the sudden. Anyway, try all that and if the problem presists, then I say go buy a dvd player or exchange your ps2.
v7 NTSC PS2 Duo2 SE