I do not know much about beta blockers, I do know that for some, they are a true "God-send" and for others, they are useless, I would think it depends on the individual and how their body reacts.
You could very well be experiencing SVT, what you have written sounds like some of the articles I have read about them, and, it might not hurt to ask for another even monitor, maybe this time, it will catch one in the act. I would also do a search on this forum for SVT's, there, you will find lots of folks who have them, and maybe you can correlate your symptoms with theirs, although everyone is different, it still might be of some help.
That is all I know to say, wish I could be of more help.
Take care and God bless