I had the exact same problem. I'd drop 40% after a 10min call. I called HTC and they sent me a new battery. Ironically, i did a battery pull reset right after I hung up and it fixed the problem completely. Now I have an extra battery that I carry to swap if needed. I posted in the forums about this and someone suggested Watchdog to monitor for rogue apps.
Sent from my HTC Evo 3D using Tapatalk
Sent from my HTC Evo 3D using Tapatalk