Javascript objects, constructors, intervals , servos and erratic behaviour

As DrAzzy says, you need:

var self = this;

not:

self = this;

That's almost certainly your problem... Personally though I'd also skip defining the small second function, and do:

var self = this;

...
  this.interval = setInterval(function ()  {
    digitalPulse(self.pin,1,self.pulse);
//    print(self.pin);
  }, 20);

That's the pattern we tend to use for modules, and while it involves writing self and not this, it is a bit more efficient both in speed and memory usage.