I'm currently having investigations for this.
I'm on my 2nd cardiologist and I am currently wired up to another monitor (for the third time). My first cardiologist seemed to be saying "it's all ok nothing to worry about" and almost palmed me off, but this new one seems to be thinking "lets get this sorted out".
I have a very slow heart rate (I actually measured 41bpm last night in bed !!!! its usually low 50's) and every so often my heart skips a beat (atrial fibulation or ectopic beats).
The biggest issue I have is that my heart rate drops like a stone after it has been raised and a couple of times when I have stepped off my bike and sat down, I pass out (after a couple of time trials and once when a mate punctured). So tonights job is to try and bring that on whilst wearing the monitor so they can see whats happening. The cardiologist wanted me to do it in hospital but the hospital had other ideas it seems.
I too find my heart beats slower/pounds when I sleep on my left. I also find that after a few drinks my heart is worse. Although my 'new' cardiologist said that the links between caffeine and alcohol on your heart beat are overstated.
It seems like a common issue tied to being older and fit (I'm only 45!). My cardiologist agreed when I joked that I was too fit for my own good.