I used to get this quite a bit Lee, over quite a period, mainly while resting. The lack of energy and tiredness is caused by the reduction in blood flow due to the heart's inability to pump properly whilst beating irregularly.
Seriously worried me at the time, and had various tests, ECG traces etc, and found nothing "abnormal" as such.
Advice at the time to alleviate it, was some sort of vigorous exercise - reason being to force the heart into working and getting into a normal rythm again, something like running up and down stairs a couple of times.
In the last couple of years I've had hardly anything, and no apparent ill-effects.