I've been following your post to see what experience would be shared. Sorry to see none has shown up yet, and your post is about to drop off the front page.
From my limited knowledge it occurs to me that you may need a pacemaker. This is very uncommon for a person of your young age. Did you doctor express any concern that you heart might just stop? I am not trying to bring up anxiety, but that is a question I'd ask if my heart rate jumped around. I guess given the low point is a healthy 60 bpm, as stop may not be a concern.
Wishing the best outcome,
I did talk with you a little bit in your other post. If I'm reading this one correctly, you're asking some "why" questions. Why does the heart suddenly go from 60 to 120? Or how does the second foci take over? I've honestly never thought about it. My heart will jump from 65 to 190 in a heart beat, race along for a bit and then drop back to 65. It's called PSVT. Why does it do that? I don't know. It just does. . . .
I thought you also had a question about IST. Any more information on that?