You don't state your age or gender or how long you have had the tachycardia and whether any medications other than Atenolol have been tried. I am not quite sure what is meant by "idiopathic sinus tachycardia." Have such conditions as hyperthyroidism (overactive thyroid gland) been ruled out?
Atrial premature beats and paroxysmal supraventricular tachycardias may be seen in the same individual and a brief pause after the premature beat is to be expected.
I would agree that one has to make the correct diagnosis of what is occurring, and a 24 hour ECG study should help. Causes for persistent tachycardia should be ruled out. Assuming the premature beats are of a benign variety, other treatment modalities might be tried.