SNS is right, CO indications should be taken seriously and intermittent CO warnings may be symptomatic of a slowly developing problem.
When training, there was one particular aircraft which made me feel slightly 'off colour' after using it and later gave my instructor a splitting headache by the end of the lesson.
The CFI flew it and the engineers prodded it two or three times and they all swore it was fine. Then one day the CO card went to solid black during a power check and eventually they discovered that there had been a developing crack in an exhaust manifold.
Personally, like any other odd indication on an aircraft, I would get it checked out.