Why does my microwave keep stopping mid-cycle?

Don't assume it's the door latch or timer right away - most often it's actually the magnetron failing or the high-voltage capacitor going bad, which makes the unit shut down as a safety thing. If it's stopping consistently at the same point in the cycle rather than randomly, check if there's something arcing inside (like a metal object or chipped interior coating), but if it happens no matter what you're heating, you probably need the magnetron replaced.