I have a 2004 honda civic in the winter time when I am driving below 30mph or sitting at a light my car 'overheats' and the heat stops blowing out and it blows out cold air but when I start going again over 30mph it goes back to normal. I have taken it to shops and its not really over heating I don't know what to do...

What do you mean: "overheats"? Needle into the red? Do the cooling fans come on at all?
It sounds like not enough coolant is moving through the system, either due to congested or clogged cooling tubes in the heater core and/or radiator. If so, flushing the system now won't help.
You'll need to determine if the problem is restricted flow or perhaps the waterpump impeller has eched away the impeller's blades.
How many miles and what does the maintenance records look like?