When I turn on the heater the gas comes on in the furnace the fan in the furnace powers up and than shuts off then the gas in the furnace shuts off I reached in the furnace and turned the fan by hand and sometimes it will go through the cycle and the heat comes on and the fan will run for 30 min. and then shuts off without reaching set temp.

How do I get furnace heating fan to stay on to heat the house ?
When I turn on the heater the gas comes on in the furnace the fan in the furnace powers up and than shuts off then the gas in the furnace shuts off I reached in the furnace and turned the fan by hand and sometimes it will go through the cycle and the heat comes on and the fan will run for 30 min. and then shuts off without reaching set temp.

If you know how to bypass the thermostat circuit for heating (usually "R" & "W" for gas heat), it would be an easy first step in finding, or eliminating, a suspect...

What make is the furnace? Model and serial number would also help. This could be many things. Here's a few, bad capacitor, bad motor, bad fan control or mother board, loose connections bad wires, etc. Can you tell us how old the furnace is. This will give us at least an idea of what kind of fan control you have. If its an extremely old furnace, pics of the inside of furnace with covers off may also help.