2007 Dodge Caliber Shifts move drive to neutral

I just bought my car from a private party 2 weeks ago. i noticed after a week that when i put the car in drive, if i pass a bump it shifts into neutral on its own.. i need to know what the problem is an how much would it cost to fix. it doesnt do it all the time just when i pass a bump or pass by a hole. i have a 2007 dodge caliber with only 81,000 miles. please get back to me asap..