Hi Nick,
Nice photo! Is it the same when the engines up to temp? If I have mine on Auto the fans speed seems to be slower until it's fully warmed up.
Griff
I'm not very good with electrickery, hopefully someone will be along soon. Could it be mechanical - I'm thinking something inside the fan stopping it spinning properly, although this usually is accompanied by a noise. Mine did when I had some of the bulkhead filter foam in it!
Griff
what happens if set the blower to maximum speed? is it responding to the setting or the speed is constant no matter what you do?