WOW! I have the same thing in one kitchen cabinet. There is a bad smell coming from that one area. Sometimes it is really strong, sometimes not.
I wish I had a good answer. Here's what we are trying: my husband went above the ceiling in the kitchen and looked around at the duct work (thinking some small animal died in there...), I have taken everything out of the cabinet and wiped it all down with ammonia and let it air out. Have looked around everywhere possible without taking the cabinet off the wall, but that is next. We might have to take the cabinet off and see what's there.
You could also try one of the remodeling / restoration type of businesses. Those guys that do home repair after water, mold or fire damage. They probably have smelled it all before and can tell you what it is....hopefully.
We thought of doing that, too, but our smell comes and goes unpredictably.