I didn't have your high reading but my MAP didn't read correctly when the engine was off. With the altimeter set to zero the MAP should read the same as the altimeter setting. Mine was off by almost 2 inches. I called Van's to see if they knew how to determine if it was the sender or the gauge and they told me that there was no way to determine where the problem was and that I should return both units.
I opened the gauge and found an adjustable Pot which provided a means to set it to the correct reading. It has been working OK. It sounds like your problem is more serious.