Have you tried omitting any pilot hole and just going straight in with the 12 mm bit. Sometimes stalling is partly due to the drill cutting right on size so the lands rub a lot. This is made worse if the drill has been well used because the lands near the tip are worn to a smaller diameter than further back.
When the drill goes in directly without a pilot it tends to cut a bit bigger so you don't get the rubbing. You can deliberately make it cut a bit oversize by grinding the cuttings edges slightly unequal.
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