Strange "framerate drop" using with my own timer class

I wrote a small timer class for use in a graphics engine I am working on
  for teaching myself.

The small time based animations I tried with it seem to work fine but
displaying the frame rate I noticed a certain "drop in framerate" to
appear at regular intervals without any apparent reason.

Here's the code I'm using:

class BasicTimer
{
public:
     void update();
     float getTimePerFrame() const;
     BasicTimer();
     ~BasicTimer();

private:
     // Copy constructor and assignment operator
     // Declared private for copy protection
     BasicTimer(const BasicTimer& rFrom);
     BasicTimer& operator=(const BasicTimer& rFrom);

     // Attributes
     float timeOfThisFrame_;
     float timePerFrame_; // In seconds
     float timeOfLastFrame_;
};

inline BasicTimer::BasicTimer()
: timeOfThisFrame_(0.0f),
   timePerFrame_(0.0167f),
   timeOfLastFrame_(static_cast<float>(clock()) /
                    static_cast<float>(CLOCKS_PER_SEC))
{
}

inline BasicTimer::~BasicTimer()
{
}

inline void BasicTimer::update()
{
     timeOfThisFrame_ = static_cast<float>(clock()) /
                        static_cast<float>(CLOCKS_PER_SEC);

     // Prevent overflow (however unlikely considering the data range)
     if (timeOfThisFrame_ - timeOfLastFrame_ >= 0.0f)
     {
         timePerFrame_ = timeOfThisFrame_ - timeOfLastFrame_;
     }

     timeOfLastFrame_ = timeOfThisFrame_;
}

inline float BasicTimer::getTimePerFrame() const
{
     return timePerFrame_;
}

I display the framerate in my main() by calling:

std::cout << (1.0f / timer.getTimePerFrame()) << std::endl;

and I get either 1.INF usually since I am only drawing two triangles
right now, or some value around 60.0 if I activate VSynch on my GPU's
driver.

Anyway, every 10 frames or so that framrate drops. Either from 1.INF to
60, or from 60.0 to 30.0 depending on what mode I am on.

It's really happening at somewhat regular intervals but I don't see
what's causing it. The animation seems to be smooth enough so the values
seem to be correct, but why do some frames at regular intervals take
longer to render?