I misunderstood you. I thought you stated that when you exercise it all of a sudden goes faster than it should. What you describe with the rate is normal. It is likely that you are experiencing pvcs or pacs but you will only know for sure what it is by seeing a cardiologist and having it captured on a monitor. That said, if your heart is healthy and you are only having an occasional one then they really don't give them much thought. If you are noticing they are worse with exercise then make sure you drink lots of water and try to ease into the work out instead of going full out right away and see if that helps. Consult with your doctor but I suspect you will be told to not worry.
It is going at least 120bpm and it rises gradually as the exercise intensifies. It is almost as though if I take a big breath and hold it for a second, then I feel the sudden stop sensation and restart. I was told that holding your breath actually causes PVCs and thus it is normal.
There are about 4 or 5 different things it cold be. The most astute cardiologist in the world could only speculate based on a verbal description. It would have to be caught on an EKG to understand what it is.
Do you know how fast it is going? When it goes fast does it seem to start going fast all of a sudden or is it a gradual rise in speed? It sounds like some sort of svt issue that is being reverted by an ectopic beat but you won't know for certain until you get a monitor to catch what is going on. Go see your doctor and/or cardiologist to get your heart looked at. In general most heart rhythm issues are not a big danger in an otherwise healthy heart. Some, like svt can be fixed but others like ectopic beats we need to learn how to deal with but the first step is to get checked out by a doctor. Take care.