2000 Lincoln Town Car Blower fan varies by itself

The blower fan on vent goes down and up by itself once it is turned on. Example: If the on vent is turned on and the blower fan set at the lowest level, in a few seconds--8 - 10--it begins to increase in intensity to its highest level. The noise interferes with me hearing the radio and is an annoyance. The vent air blows too loudly. If I turn the control up and then down it seems to modulate and return to a lower fan level, then in a minute or less resumes a higher fan velocity.
Can you advise me?

Thank you.