That code concerns the reading from your MAP (manifold absolute pressure) sensor. P0108 says it's too high. Because your intake is under a vacuum, which is negative pressure, that reading indicates that you are losing vacuum. (i.e., high reading means low vacuum)
Of course the sensor could be bad - but, when they go it's usually not intermittent. I'd look for cracked vacuum hoses or a malfunctioning IAC (that's the little "cylinder" directly down-stream from the throttle body).