Hvac fan runs and shuts off in a cycle when in heat mode
I imagine you are referring to Heil Model N4H336GKE100.

Check the test control jumper on the defrost control board.  In the "speed up" test position it will cause the timing you describe.  For normal operation the test jumper should be OPEN (no jumper or metal object shorting the test pins together).  The default for the timing jumper is usually "60."  For dry climates the timing jumper should be "90".

The tech support manual is posted at this address:

http://icpindexing.toddsit.com/documents/086477/42804510201.pdf

Here is a snapshot from the relevant section: