[lm-sensors] Reported fan speed oscillates 730 RPM -> 337500 RPM/675000 RPM

[simon]

Hi,
The following output shows periodic output from sensors for my CPU
cooler over a short period.

The value continually oscillates between a real value (around 720-750
RPM in idle) and out-of-range values 337500 RPM/675000 RPM. When the
machine is loaded, and the fan spins faster, the fan value stabilizes at
a normal value.

The fan 2 sensor has no problems.

Anyone have any ideas what's happening?

Configuration: 
lm-sensors v3.01 
Motherboard Gigabyte GA-MA78GM-SH2 
CPU Cooler: Thermaltake TMGA3 (3 wire)

Output from sensors over ~2 mins:
it8718-isa-0228

fan1:          726 RPM  (min =    0 RPM)
fan1:       675000 RPM  (min =    0 RPM)
fan1:       337500 RPM  (min =    0 RPM)
fan1:          738 RPM  (min =    0 RPM)
fan1:       675000 RPM  (min =    0 RPM)
fan1:          726 RPM  (min =    0 RPM)
fan1:       675000 RPM  (min =    0 RPM)
fan1:       337500 RPM  (min =    0 RPM)
fan1:       675000 RPM  (min =    0 RPM)
fan1:          725 RPM  (min =    0 RPM)
fan1:          725 RPM  (min =    0 RPM)

Many thanks for ideas!