This sounds similar to a problem I had with my '88 F-150. I think what your problem is, is that the actuating rod that connects your key tumbler to the ignition switch is binding from lack of lubrication. The very best way to fix this would be to remove the plastic cowling from around the steering column and take a Q-tip and put a light coating of grease on the actuating rod. An easier way that might help would be to take a WD-40 type product with a straw on the spray can and spray in there at the correct angle.