Just a thought, though I'm no doctor and my experience had similarities but wasn't the same. I kept getting the fluttery heartbeat and skipped beat, usually followed by a catch up beat. It was occuring whether I was running or not. It was not long after I'd started work somewhere where I was having more hot drinks than previously.
To cut along story short, after googling I decided to cut down on caffeine and start drinking decaff drinks. Bingo. No more problems.
I used Lucozade ones in my first mara - only had 3 or 4 and they were fine. Had around 8 or 9 in my second mara and felt ill for the last 10 miles Didn't use any in training, so pretty much serves me right.
Train with different ones until you find one you like and then stick to it!
I definitely recommend Dublin. My time there was half an hour quicker than my London time 18 months later, mostly down to the nasty heat in April compared to the cool loveliness of Dublin in October.
I stayed out in Ballsbridge at one of the D4 hotels and walked to the start line - it's about 25 mins walk which I found perfect for getting hydrated and warming up the muscles.
The whole thing was really well organised. One of the best things I did was meet up with the Dublin hash the day before for an easy run round the town and also going for a nice Italian with them them that evening. I haven't read this whole thread but I suspect Liam from the hash will be on here at some point if not already to invite people to join them before this year's race.
Dublin was my first mara and I loved it. There's no real hills as such, just a few gentle gradients but nothing long and there are good crowds to keep you going. There's a bit of uphill after about 1-2 miles but you're still pumped on adrenaline so hardly notice it. At about mile 8-9 there's an uphill after you come out of Phoenix park but that's the hardest bit as far as I can remember, though I may have blocked worse out of my head