93 300SD Ignition won't turn
 

We have had the car for about 2 weeks. Sometimes when you turn off the engine and leave the key in the accessory postion then turn off the key to restart, the ignition will lock and not move. We tried removing the key, locking doors, getting out and reentering, nothing works. After about 3-4 minutes it releases and works just fine. Is this part of the security system, or is something broke? I appreciate any help.

Ben