Ceiling – Why has the ceiling fan’s speed become slower

We have owned a Minka Aire ceiling fan for about 4 years and it has been great overall; running for 24hrs a day.

It has 3 settings and when we initially purchased the fan; the speed was noticeable and the air movement was as well. It now seems as though the highest setting is not much different then the middle setting.

What would cause this and how can I get it back to where it was in terms of performance?

It comes with a lifetime warranty on the motor; however I am not certain that the motor is failing here. In researching this it appears the cause could be the need to oil the motor; however I want to get verification from the community before I take on that effort.

UPDATE:

Manual rotation produces no shaking or wobbling, they move very smoothly. If you are not familiar with these fans they are fairly heavy duty, not in any way unstable as would a typical $49.99 fan be that was picked up from a local Home Depot.

On occasion there is a humming that occurs, it is not consistent by any means and is fairly rare. In addition it will sometimes make a noise as though it is off kilter slightly, hard to describe. If I move back down to the medium setting, the noise subsides. Sometimes I will wait 10 – 15 minutes, turn it back to the highest setting and it functions as expected again, minus the overall speed. This is rare and will happen maybe once or twice a month. Keep in mind the fan runs non-stop, 24hrs a day.

UPDATE 7/27/12:

This issue still exists. It now appears that the fan operates at roughly the same speed across all three settings. The instances where it sounded as though it was slightly of kilter have not happened in many months. Any ideas what could be causing this? There is a lifetime warranty on the motor however the motor sounds fine, it's the actual speed that appears to be the issue. Should I simply replace it and move on or is there something I could to to potentially fix this myself?