Try using some Brasso [liquid metal polish] on the disc to get rid of any minor scratches on it.
Apply the Brasso, then let it dry, then rub vigorously, it works like a super fine cutting agent and removes any scratches off CD/DVD's .

Don't try a substitute, you need Brasso.

Other than that, maybe it could be dirty lens of the PS2, use a CD/DVD lens cleaning disc.
OR
It could be corrupted save data on a memory card, if you have another card, copy the FUSION game saves from one card to another, then see if the problem still occurs.
It could also be a dodgy memory card slot [Something I suffer with on my old fat PS2 ], It will work up until a certain point, then won't load, I've found that just putting the memory card in the other slot fixes it.
You might find that you might need to put a wedge of paper between your memory card and the PS2's slot to keep a constant contact, it's trial and error.
It's usually because over the years the contacts of both the memory cards and the PS2 card ports have gotten dirty, so one or the other is causing problems loading from the card, which will make the PS2 think that you have still to open up that part of the game.
You could try to clean them with some Isoproyl Alcohol and some cotton buds to wipe all metal contacts on the cards and inside the ports, it will evaporate and do no harm to either.