I have had a very similar experience over several years...the SVT episodes have gotten more frequent over those years. I am a 74-year old man who has completed 74 marathons. My best time was 3.25 hours 30 years ago. My times were going down just like most articles say...about 5 minutes per mile per year. I first had issues with heart problems about 25 years ago, had many tests, treadmill, heart monitoring. radioactive stuff, MRIs, etc. I was told my heart was great...don't worry. Then perhaps 4 years ago, the events started to happen quite more often and more severe. I really pushed the doctors and finally ended up with a heart monitor taped for two weeks to my chest. I was instructed to push a button on the monitor whenever I felt the condition. By this time, I could tell when the condition was starting. Usually, when it starts, I can stop, walk, or otherwise slow until it passes. Most time it goes away quite quickly, within a few seconds. But...sometimes my BPM can stay elevated for up to 10-15 minutes. Anyhow, following the heart monitor test and pushing the cardiologist, I was diagnosed with SVT. This only happens when running, and oddly, mostly when I either slow, stop for a traffic light or water fountain or begin a downhill.
In any case, it's scary and at the least very annoying. I have continued to run races, but my times are nothing of which to be proud. I've just started to train for the winter marathons here in Southern California; it's a pain in the rear end to work around this condition.