Wierd MAP reading

[LeesTruk]

In diagnosing a driveability concern with my LS swap, I was looking at the data stream on my scanner. The MAP is showing between 11.2" and 12" at 675 rpm. Thought that was a little low for the cam I'm using, so I put my shop vacuum gauge on the nipple on the plenum. It was registering a steady 19" at the same time. Thinking I had a bad sensor, I replaced it. Guess what! The new sensor reads the same. I know the rear of the plenum is a vacuum source for the power brake booster, so I blocked the vacuum line. No change. Any ideas? As a side note, the truck goes instantly into limp mode when trying to drive. All of my peripherals seem to be okay, diadnostically. Would a bad PCM send a bad ref. to the scan reader, or should I be looking elsewhere?. I've got no vacuum leaks that I can find. Thanks