Fan speed issues

[JackIV]

The fan speed slows down intermittently on my single stage furnace while running the ac.  I have 2 furnaces and the issue follows the thermostat. The nest controls do not have a fan speed setting option.  This issue is not present with my prior thermostat (not a nest) so the issue is not the furnace blower. 

[CoolingWizard]

@JackIV ,

Do you have a multi speed fan set up on your Nest Learning Thermostat? If you have a single G wire then there nothing the Nest Thermostat can do to slow the speed down.  If, you have a multi speed system, the software only changes the speed when you select a feature like “Cool to Dry”.   
So why not share a little more so we can help you. What is your system configuration including uploading a picture of your Nest Backplate showing the wire that are connected.  

AC Cooling Wizard