Sometime they overlap, just like some people have both SVT , NSVT, PACs & PVCs, a person can have both POTS & IST, usually IST is the dominant problem , you might want do a search on dysautonomia which all these symptoms tend to occur, hope you feel better soon. Believe me I know how hard it is and what you're going through. If your beta blocker is working and causing no real bad side effects try and stick with it and don't skip doses it only aggravates an already complicated though "benign" condition.
Ooops I wrote "my resting pulse is between 60-80, which I think gives away that it might not be POTS" That was supposed to be "might not be IST"