Tapped holes keep stripping in 6061 aluminum

I'm tapping M6 threads in 6061-T6 aluminum, 1/4 inch thick. Using a spiral point tap and cutting fluid, but some holes strip when torquing to 10 Nm. I've tried slower tapping speed and a forming tap, still occasional failures. I also use a tapping guide for perpendicularity.

My drill size is 5mm for M6x1.0. Could the drill size be off? Should I use a slightly larger minor diameter or switch to thread inserts? What's the best way to ensure strong threads in 6061?

#Steel# #SurfaceFinish# #Aluminum#