Where is the melting occurring? If any metal in the oven is bare the microwaves will heat that metal. That is why the paint inside the unit is so important. One thing you can try is to remove the light inside the unit and then put a glass of water inside the oven and turn off the room light and start the oven and watch for arcing to figure out where the metal is heating up. Note that the paint inside the oven is a special paint and once arcing starts it is hard to get rid of.
Do you have a water supply hooked to it? Is the water turned on? If it is turned "on," but not flowing, did you install a one-directional supply hose backwards?
