A shutoff problem is not in the ignition switch. Assuming what you are refering to is the electrical switch and not the vacuum valve on the ignition.
It is most likely vacuum related.
Did the problem start right after an oil change? If so you likely have some vacuum lines disconnected. Check them all.
If thats not the case you need to check for vacuum leaks. Door lock system leaks can kill your shutoff. AC vacuum leaks can kill your shutoff. You might have a vacuum leak on the brake booster, or the EGR or the transmission. All these things can and will stop your car from shutting down properly. Before you actually replace anything you need to do a quick check of the vacuum system.
If you don't have a leak then you can start looking at the shutdown components themselves. Valve on the IP, valve on the ignition.
If your sure the part you have is the source of the problem, you can get some information about the dash/cluster ignition removal at
http://garnet.acns.fsu.edu/~jjs5772/W123
Thats more geared to ignition problems... but it explaines how to get the ignition out.