Their not supposed to glow all the way to the tip when testing. It only does that when its actually firing up inside the engine. If the element that sticks out is gray and intact and lighting up orangey/red then the plug should be fine.

Usually when your car is idling when you start it up and when you take out the igniter the revs change drastically, it either means your plug isnt in good condition or your plugs heat range is too cold for the engine