Maybe not the best answer, but sometimes you can just have funny little arrhythmia's that are a physiological problem. Some come on randomly, others have specific triggers. I have had two heart surgeries for an atria-ventricular nodal re-entry tachycardia, and for years mine was a just a random few bursts of arrhythmia, starting at about the age of 3 until I was 17. It usually happened when I was on the couch or resting. When I was 17, it changed- I started getting them every week or so, and a really debilitating can't-breathe-pass-out-emergency-room ones about once a month. But, even when they were happening that frequently, they still didn't show up on any monitors or stress tests. The first time they caught it on a monitor was when it was induced during my first heart surgery. This was after about 20 24-hour monitors and 4 7-day monitors, as well as maybe 12+ trips to emergency, and 24+ trips to clinics. So, needless to say, it is not unusual that you don't see anything on a monitor.
Some typical triggers I was told to look for when they were first investigating (even though they ended up not applying to my tachycardia) was caffeine, exercise, alcohol, blood pressure changes, and lack of sleep. One thing I tried with my cardiologists support, was I had a week-long monitor on and just tried to do every possible trigger during that time I had the monitor on- he called this the "real life stress test", and said it was much more effective at recording irregular rhythms than any one they could perform in the office. I went to live sports events (hello blood pressure rising), went out clubbing (drinking and lack of sleep and physical activity), drank coffee w/booze (two in one!), did sprint-rows on no sleep (tired and vigorous exercise), and just generally had a fun/tiring weekend. My cardiologist said that you won't find a lot of doctors who will tell you to go ahead and do this, but if you can get something on the monitor it could settle things for you in the long run.
My advice would be to talk to your cardiologist again, and see if your doctor might want to monitor what mine called the "real life stress test". If you and/or your doctor are not down, it probably won't change anything to get a second opinion. Just be attentive, track all the occurrences in a calender. If you can notice any common events that happened around the time you got your arrhythmia's (days where you were tired, where you had some wine the night before, etc), that could be helpful if you wish to investigate further. This will also help identify any changes in frequency.
As my cardiologist said to me, so many people have these funny little arrhythmia's, and for the most part they don't mean anything- it's just a little nerve firing, and even though it feels big to us, it can be the heart equivalent of a twitch in your foot. However, if it is disrupting your life (as mine was in a big way), it is a pretty easy fix- usually just one closed heart surgery, where they either freeze or burn the nerve pathway that is causing the disruption.
I'm not sure if yours is the same thing at all, might not even be an electrical problem (though more are, as it's pretty easy to rule out structural after an ultra-sound), but my experience was I was fine living with it, until it became an obstacle. Then, I had two relatively easy heart surgeries, and so far haven't had any other problems.