Don't even think about a new gearbox yet,look for a local garage that specializes in automatic gearboxes and ask them to diagnose the fault as it may only require transmission fluid top-up,just tell them the problem and they will find the fault,DO NOT suggest to them that you think you need a new gearbox,it will cost you the earth,advise is cheap or free.Look in your local Yellow Pages it will be quicker and easier trust-me.
Rich, January 2010