I would think you would need some code in the text setter.
Can you please elaborate?
What Flex harUi means your text-setter property of your custom tooltip component is simply empty. But that seems not to be the problem.
This row is the issue:
var cName : String = item.getRepeaterItem().sgname;
It would be easier if you send simply the text over the createCustomTooltip handler.
The you can simply set the toolip depending on the current data item.
var cName:String = item as String (cast is only necessary for the object of course)
This could also be done with more complex data.
I believe the toolTip property is assigned as the text to the IToolTip