Haha, I actually had the same question I was thinking about posting today.
You go wrong right in the first step.
delta U = q - w
The only reason it is + w in your equation is because that is work done ON the gas, not work done BY the gas.
I think that should solve your problems.
Or if you want to stay consistant, in your third equation, you are using work done BY the gas. Change this to W = -p delta V, then it would work as well.
