The RPMs jump from 2,500 to 4,500?

1995 Infiniti G20 1.8 mileage: 101,000. Automatic Transmission. I purchased this car brand new and the first signs of transmission problems began yesterday. Once the car gets up to 55MPH, the RPM jumps from 2500 to 4500 and the transmission won't budge, it tries to shift, but is unable to. The only way to make it from point A to point B is by going under 55MPH, and as you know, this won't work on the Washington DC Beltway. I have taken it to a local Infiniti dealership and they said everything checks out fine and that they test drove it. Well, they misinformed me, because when I picked up the car, I checked the mileage, and found that they hadn't driven it. They said they tested everything out on their computer and everything tested out. Help!