Might try squirting some wd-40 in the key lock assembly to see if that loosens things up, but it sounds like the ignition key switch (electrical) could be going bad. I haven't replaced the igntion assembly on a Lesabre, but from what I have read, the steering wheel has to be lowered and possibly pulled to get to it. This is unlike the older cars where you could drop the steering wheel and acess the ignition assembly for replacement. The rod runs from the key lock assembly down to the ignition assembly. I thought I may need to do this on our 97 Lesabre, but it turns out is has a different problem. The books says to disconnect the battery and pull the airbag fuse before messing with the steering wheel. I found this article on the subject:
http://shbox.com/ignsw/ign_switch_repl.html