Most thunderstorms occur in the late afternoon or evening after maximum radiational warming. In the late afternoon, instability tends to be highest due to daytime heating increasing the temperature gradient between the PBL and the middle levels of the atmosphere. Lift and moisture can be present to a high degree at any point of the day, but instability does have a strong diurnal change.
As daytime heating occurs it increases the air's capacity to evaporate water vapor. If soils and vegetation are moist or a large water body is near by, daytime heating will allow dewpoints to increase due to the stronger evaporational power of the air. Greater amounts of moisture allow for a greater amount of latent heat release in a thunderstorm and higher levels of instability (one way to increase instability is to increase low level dewpoints).
If a cap is present, daytime heating will partially or completely erode it. The late afternoon is the most likely time that a cap will be eroded to zero.