Sounds like your VATS sensor in the ignition switch is wearing down.
VATS is Vehicle Anti Theft System. Your key should have what most call a "chip" in it = it's a small black pellet - it is actually a resistor. There is a box under the dash that looks for the correct resistance before the car will start.
Inside the ignition switch, there are 2 prongs that are spring-loaded, so that when the key is inserted, the pellet on the key seperate these prongs, and thus the prongs are tight against the pellet and "read" the resistance of the pellet. These prongs wear out - the springs go bad after years and years of being used.
Best fix - get a new lock cylinder from Carquest (about $50), and replace the lock. The new lock will come with a plastic blank, so take it to Ace Hardware with the original key, and have them cut a new set of keys using the new blank (plastic key) with the resistance from the old key.
Or, you can bypass the VATS altogether - read here:
http://www.thirdgen.org/vats
and
http://www.thirdgen.org/vats_passkey_system