Loss of power, gear box overheating

I believe that van has the Nissan CVT in it, they are a very failure prone transmission. The only way to accurately diagnose them is to use a scan tool to test the transmission. In your case it should have stored codes. From the codes it would tell you if it was in the control unit or much more likely a mechanical failure in the transmission itself. If it the transmission you may be still under warranty and can get them to replace it. Nissan has had a lot of problems with the CVT and have extended coverage on them in many countries.

What is the meaning of CVT?

Constant Velocity Transmission. It is basically two pulleys riding in an oil bath with a steel and polymer belt between them. You should have a second generation Nissan XTRONIC CVT in your van.