Often with a manifold pressure gauge exhibiting this behavior, a little grease has entered the bourdon tube which connects the instrument with the induction manifold. The pressure trapped in the instrument creates a higher than normal reading.
Sometimes mechanics will use teflon tape when sealing the "b" nut on the back of the instrument; this is a common, but poor practice. The tape can work out of the threads on the b nut and cause a blockage. Generally the first step in trouble shooting this is to disconnect the manifold pressure line at both ends, and blow it out using shop air from the instrument-end. If this doesn't correct the problem, then submit the instrument for repair work.