Sorry, left a word out... meant to say PT6-TYPE engines, ie engines with the air intake below the spinner and a plenum chamber leading, via by-pass vanes, to the engine air intake at the rear of the engine (kind of a PW trademark really).
The culprit here is a buildup of ice or snow inside the intake plenum... obviously hot lips won't fix that. It can be checked visually, or by sticking your hand in through the by-pass doors and feeling for contaminants.