What Causes Fluttering Sensation In Chest While On Atenolol?

This has been going on for about 10 years (I m now 62, male, healthy, not overweight, excercise in moderation, non-drinker, non-smoker, do not drink coffee or tea, on 50mg atenolol daily for hypertension). I suddenly have that fluttery feeling in my chest and my heart will skip a beat, then beat more strongly and resume beating normally. It can happen without warning and an episode can last an hour or more. Sometimes it can go on for days, other times I can go a month or more without it happening. Sometimes for about a minute my heart will beat 3 times then pause, or 4 times then pause, then resume normal beating for a minute or less, than it resumes. It s unpredictable. I ve talked to my doctor about it, had a stress test, all normal. Of course, it never is happening when I m at his office. Any ideas? This must be a fairly common complaint.