Not able to get high a/c fan output while driving.

When I start my 2011 Prius I can usually get the highest fan speed going while it's in park but once I start driving it will put out air closer to a low or medium speed and I can hear/feel it fluctuating up and down. If I put the car back into park I can eventually get it to put out high speed air but it takes a few minutes. I suspect it's a bad blower motor and/or resistor but the fluctuating up and down only while driving isn't a symptom I'm seeing others experiencing. Any idea what this could be?

 

I should add that after about 25 minutes of running, the A/C seems to work normally.

 

And sometimes if I set the fan to the lowest speed for about a minute, the issue will be resolved.