Spec, brand-new:

Athlon II 240 AM3, retail version
Asus M4A78T-E

That's probably all that is relevant. When I cold-start the machine, about 50% of the time, the fan blades budge about half a centimetre and back to their original position, about three times, then it'll come up with a CPU fan error, then the fan will start spinning at the desired speed. I've had this configuration working fine before so it has me puzzled. I've tried a BIOS update for laughs, and I think the problem doesn't happen if I don't enable Asus Q-Fan on the CPU fan, but the system is significantly more noisy without Asus Q-Fan enabled, and I've had it working fine on identical PCs before this one.

Any ideas? I tried swapping the chassis fan and CPU fan connections, which works, but I don't think the system gets cooled in a desirable fashion when under load.

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My PC spec