The parameter to tune the spacing between axis marks is the MaxLength parameter of the Intervals property of the axis - this is the maximum distance between adjacent ticks (in pixels). When the size of the chart increases so that the same tick distance would increase the axis switches to the next unit as specified by the NiceSteps values.
This works also when the default axis scaling is bypassed by using the DateTimeIntervalChartSource. In this case, the property with the same effect is called "Params". So, in summary, you must increase the Params.Maxlength property of the DateTimeIntervalChartSource to something like 100 (default is 50).
Alternatively you can also rotate the labels.