Going to low fan to high fan back to low fan back to high fan within 3-4 minutes known as heaving.

Yes your system has not been adjusted properly by the installer OR someone messed with the settings. Call the installer back and have the settings adjusted properly or that expensive motor/controller will be toast.

It has been working for years and it just started last week or so.

Then make sure all registers are open and that air filter is clean with no other blockages of the return air grills.