If all tests are normal, then you have a structurally normal and heart and have nothing to worry about. Irregular heart beats or "skips" mean nothing in such a heart. Most folks have these skips they just do not notice them, they are very common.
If they continue to worry you, I would see you doctor, just for the peace of mind, reassurance is everything.
Are you sure it blocks for a second an not just a beat? Extra beats arising form odd areas of the heart can disrupt the normal beat cycle of the heart and lead to a pause as the heart resets itself but if they are isolated then they are nothing to worry about. A lot of people get them. If you are not experiencing any troubling symptoms like shortness of breath, lightheadedness, chest pain or fainting there really isn't anything to worry about. Try not to worry about it because anxiety will make whatever is going on worse but if it does get worse go back to the doctor. But since your tests all indicate a structurally healthy heart you really don't have anything to be concerned about. Take care.