It sounds a lot like PSVT to me and I've had over 50 years experience with that. The best way to see what type of arrhythmia is happening is to wear a heart monitor. If it happens every day, then a Holter monitor is fine for 24-48 hours. If it happens occasionally, an event monitor would be better. Typically worn for 2-4 weeks, a person can go about their business and if the heart acts up, they just push a button to record the event. Pretty simple and the recordings can be sent over the phone.
The straining as for a bowel movement is called the Valsalva maneuver and can be quite effective in converting the rhythm back to normal. I've used it for years. Before I learned about Valsalva, I did as you do - had to lie down till it passed.
Do press your doctor to check this out. There are ways to manage and treat PSVT.
ireneo is right, everything you have done I to have done as well to stop the attacks.
I was born with PSVT and still have it, I have attacks were my heart rate goes over 300 bpm. Don't let the doctors blow you off. If you don't feel like they are doing a good job, find another one until you get the correct answer.
I manage my PSVT with Propranolol 20mg twice a day and Clonazepam 0.5mg twice a day. And so far so good.
As for the dieing part, on the normal, no you can't die from it one doctor told me.(And I have had attacks that have lasted 22-20 hours at a speed of over 300 bpm.)
But as far as I am concerned, PSVT is nothing to play around with. And make sure you don't take anything, and I mean anythng that is Adrenaline based, check your OTC meds, to make sure they won't speed up your heart rate, and if you are not sure, check with someone at the Pharmacy, they will know.