This usually happens on a "bad" burn of a game. But you did say that the game used to work, so that leaves us with another problem.
I am not a tech expert, but this is what I am guessing the problem is: You laser in your Duo is going bad.
The best way to fix this is to send it to retrogames and let them fix it for you. Or you can check out Turbo Zone Direct and just buy a new one.
Now before someone tells me I am 100% wrong, please keep in mind that I am not a tech expert, and have had this problem once, and upon receiveing a refurbed Duo from TZD (When they were still doing warranty work) everything worked fine.