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So about twice a day I am finding my furnace is blowing cold air. what is happening is it is running just fine and them the jets shut off but fan keeps going thus blowing cool air. If I turn the furnace thermostate down so it shuts off then turn it back up so it kicks on it runs fine....well for half a day or so then again jets cut out and fan keeps going. Any thoughts????  

faulty low limit switch

thank you do you work for a furnace company?

Change your furnace filter. Furnace is starving for air and shutting off on high limit. Fan continues to run to cool off limit switch and fire box.  
Change your filter and you could be fine

When is the last time you had someone come out to clean and service your furnace?

It could be a number of things depending on the type, make, model, and how old the furnace is. New furnaces has a lot of safety switches in the to prevent gas turning on(that's why there are far less gas fires then electric fires). It would be best to contact a heating company to have a look at it.

99sportster said "Change your furnace filter. Furnace is starving for air and shutting off on high limit. Fan continues to run to cool off limit switch and fire box.   Change your filter and you could be fine "

Listen to Sportster. This stuff is his profession.

I assume this is a gas furnace? Mine was doing exactly the same thing a couple years ago. The fix was cleaning the flame sensor. Every year when I change the filter, I clean the flame sensor and I haven't had a problem with mine since.

Yeah clean your flame sensor, yes check your filter. If you have a Honeywell gas valve there's a good chance the circuit board needs to be fixed. But if you are on here asking for a quick fix, you need to do some googling and some reading. And if you are not comfortable or at all mechanically inclined, please call the "guy" . Furnaces are not that technical, fairly simple modules. But you need to know what you're doing. Best of luck.

Is your exhaust vent.  
 
Our former furnace was of poor design and the exhaust vent would constantly condense and plug, which caused the furnace to stop heating though the air would still run. We'd have to defrost the blockage and reset the furnace a lot.  
 
When we put in a new one, we rectified the issue with a new vent that is of better design.