Hello Martin, and welcome to the forum!
I'm in your age group and have also experienced some heart palpitations recently, some which have been lengthy, usually noticed at nighttime. I haven't been to a doctor about it, but it is disturbing to say the least.
From the little I've read about the condition you mention, they say that reducing or eliminating caffeine does not effect the symptoms either way. Interesting that your magnesium levels are normal, that the first thing I thought may help. Here is a page that suggests Taurine and L-Arginine for treatment...
https://george-eby-research.com/html/...rhythmias.html
I've taken sleep blends with Valerian in them, but never the herb by itself. I heard that it will slow down the heart beat, and I didn't think that sounded very safe.