2002 Chrysler Town and Country Rear power lift gate problem

When the temp. Outside is cool, or even the ambient summer temp, the remote nor the inside power button will not opeate the rear power lift gate. After the sun shines on the lift gate and warms it up, both the remote and the inside button opertate the lift gate perfectly.
The dealer says they "think" the problem is a short in the motor even though they did could not find a short. Any ideas on the problem?