Hi, I also have POTS and I had an ablation done in 2001 for AVNRT and IST. I have similiar symptoms that you are having. My heart will just out of the blue start racing and then I feel a hard thump and it goes back to normal, all within seconds this happens. I haven't been able to catch the episodes on any monitors so I can't say exactly what is causing it. It is truely frustrating after being so hopeful that the ablation would take care of any tachy. I never had this kind of symptom though before the ablation so I assume it was caused from the ablation. How long ago did you have your ablation done?