Power Assist Brake problems of the 1997 Dodge Grand Caravan

The power steering/power brakes have been locking/failing intermittently for the past year. Took the car to the shop in July of 2001 and was told the steering wheel problem was due to humidity and loose belts and the weather would improve the performance. Noticed that when the steering locked, I was driving the car between 5 and 10 miles per hour. Also, when steering wheel locked, power brakes would fail. Had to literally stand on brake pedal or turn off car ignition to stop car. Threw car in park and turned the ignition immediately to regain steering and brakes. Had no steering capability at all. This problem has happened about once every 2 months but seems to be getting more frequent with 3 failures in the past 2 months. Took car into dealer on 6/24/02 and they worked on it for a couple of days before deciding the problem was some part of the steering column. They could replicate the problem with the steering but not with the brakes. They replaced some component of the power steering and released the car 6/26/02. Picked up the car on 6/27/02. The next day, 6/28/02, both steering locked and brakes failed. Will have car towed to shop today. Both son and husband have also experienced this problem while driving. Son nearly hit a tree when steering locked. Said "check oil light " came on dash. Said the only way he could stop the car was the turn the ignition off while in drive. We had noticed the problem more when making a turn. Husband said the problem happened to him this past Friday (6/28/02) and he wasn't turning at all. We do not see "check engine" light flashing on when this problem occurs. We are all scared to drive the car because we are afraid it will stall or the steering wheel and power brakes will fail to work at high speed.