MAP is measured via a sensor on the throttle body. So most likely causes are a dirty or defective sensor, or air leaks at the gaskets where the intake manifold meet the cylinder heads. There have been a number of reports of leakage at those manifold gaskets which often coincides with using an aftermarket air cleaner without support at the front of the cylinder heads. Air leaks are most likely on an older or higher mileage machines. The gaskets are not difficult to change, but it does involve removing/disconnecting pretty much everything between the cylinder heads (fuel system, throttle cable, sensor connectors, etc).
In any case, I would be inclined to look for air leaks first, then failing to find any, I would remove the air cleaner to use a bit of carb cleaner spray on the sensor, as well as to unplug then replugin the sensor.
It's helpful to those of us trying to help you if the year of your machine was in your signature.
