Diagnosis
You're overloading the motor. The NutriBullet 1200 is powerful, but it's meant for short pulses, not for running continuously on very thick mixtures like nut butters or super-frozen smoothies.
How to Fix It
1) Cut your blending time. Use the pulsing technique: hold the cup down and press the button for 10-15 seconds, then let go for 5-10 seconds to let the motor cool. 2) Watch your ratios. Add more liquid to your recipe. A good rule is the liquid should at least cover the blades before you start. 3) For frozen items, let them thaw a bit or use the smaller cup to keep the volume lower.
If That Didn't Work
If the burnt smell is persistent even with light use, the motor's thermal fuse might have been damaged. Unplug the base and let it cool completely for at least an hour. Try it again with just water and ice. If it still cuts out or smells, the base unit likely needs replacing.
Heads Up
That burnt smell is the motor's insulation overheating. Pushing it too hard too often can permanently kill the unit. If it's under warranty, contact the manufacturer. They're pretty good about motor issues.