Ignition problems

I recently purchased a 2001 Buick Regal GS with around 85000 miles on it and have encountered a ignition problem. On occasion the key will not come out of the ignition, it will go back to the past the "on" position but not to the "lock" position. It seems to get stuck right before the "lock" position. As I said, this only does it on occasion, other times the key comes right out without any problems. The car is definitely in Park and I have tried wiggling the gear selector to make sure it has nothing to do with this. I am stumped, any help on this matter would be appreciated.