You may have an undersized (or a restricted) furnace or the winding of the
blower at this speed is becoming weak, basically, you should increase the
airflow to pass across the heat exchange, you need to do a CFM calculation
before you correct it, check the temperature rise between the return duct
and the supply duct, use this formula
CFM = BTU* 80% OR 90% /(1.08* TEMPERATUR RISE)
After you have the CFM, if it is far below the nameplate's, you should check
the restrictions from all components equipped in the duct, such the evap.
coil. All dumper must be fully opened and the bypass damper must be closed
when you do the test.
The gas pressure may be higher than 3.5", it should be checked and correct
as well.
The pressure test of the duct system should be checked.
A quick to solve the problem is to change the speed tap to a higher speed at
the blower, it is easy but may hide the issue of the duct system.
If you think it is too complicated for you, find a professional.