Ignition Switch problems of the 1999 GMC Safari

On 4 occasions, approximately once a month, the key has been inserted into the ignition and failed to turn. On 3rd occasion, dealer found shifter cable intermittently getting stuck, and not allowing ignition to release. They also found case jamming up. They replaced steering column assembly. Currently car is at dealer for the 4th occurrence. They are suggesting that our driving/parking habits are perhaps to blame; we feel that, as mature, good-driving adults in our late 40's who have driven many types of cars and vans throughout the years without ever encountering this problem, - that there is no way we could be causing this problem. Obviously the mechanism in the ignition/steering column is highly sensitive, and definitely has a defect. My husband and I fear that we are someday going to get stuck in a far-away place or in the middle of a job with a car whose key will not turn. The dealer cannot find the problem, and I wonder what they are planning to do for us.