So, here's two things that worry or intrigue me about the Stitch central mechanic:

1. Failure's too arbitary. Failure in the final seconds could be down to whether the GM brings in your doubt or not.
2. Connected to 1, I think it needs some drama, some sparkle. At the moment, success or failure is all down to adding numbers. Some randomness would add drama (or, failing that, something like Polaris' "But Only If..." mechanic).
2. I wonder if you could combine it with the whole Inconsistency mechanic?

Here's what I'd do, off the top of my head.