Bosch 1617EVS speed problem

While I was using my Bosch router today, model 1617EVS, I noticed that the sound of the router was varying both in use and when simply turning. I'm thinking the speed is not staying on the one I've selected.
Anyone else have this happen? Is this a known problem with this router?
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The sensor might be optical, so hose 'er out with some clean air and see if it doesn't help. I do mine periodically, and haven't hit a problem ... yet.
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