HR will be at 190 then in one beat drop to 170 then rise back up. Keeps happening a few times in exercise. Short of breath, extreme pounding.?

Don't rely on HR. monitor. These are not accurate enough to determine the rate just on beat to beat variation. If you are however experiencing extreme pounding in your chest, and are short of breath, ESPECIALLY if you feel at all woozy or weak, you should get medical attention. At 17 probably not worrisome, but never can be sure. Avoid extreme exercise until this is done. And don't diet.