the central air inside blower doesn't always work in fan ON mode only, it blows fine with cool/heat mode. When in fan ON mode only it would sproadically go on and off, pretty much random. I thought it was the thermostat issue, replaced to a new one still same issue. Ideas?

The newer system use a 3 speed blower. High speed for cooling, Low speed for heating and lowest speed for fan only. You may have a bad connection on your blower control board. A make and model of your system will help solve the problem.

Thanks for you reply.

It is made by goodman manufacturing, model no. h-60f