If anything the big cog helps you get more contact with the chain and will help you hold your chain. And if the line is straight, my guess would be your tensioner. They are really hard to set up right, you want it as close to the chainstay as possible, with the spring set pretty loose.

But you dont mention a chain tensioner, which could be an issue. If you have vertical drop out(which is probably the case) you should be running a chain tensioner, even if you try to run a "magic" ratio.