[PeteA216]

Since it's an original key it may be worn and isn't fully engaging the tumblers inside the ignition. Do you have a copy laying around anywhere? Or even another original? If you do, try one of those. If it still gives you trouble even with a different key, the problem lies within the ignition itself. If this is the case, then as long as you don't have the little chip on your key, try squirting some penetrating lubricant into the key hole. Sometimes things like to stick. Worst case scenario would be that you have to replace the ignition, which is actually a fairly simple and not-too-expensive thing to do assuming you have access to a steering wheel puller.