Well hi there! It's funny that this post came up in a Google search for the same issue I had back in December 2015. I was reading the post and didn't even realize that it was me who'd originally written it. My wife was reading it with me and said, "Boy, that guy sounds just like you!" And then, sure enough, it was me!
Whatever. I thought it was funny.
Anyway, so it's been quite a while since I cleaned out my old torrent tasks and I've got them all removed, except for one. The same one that plagued me back in December '15. Simply highlighting this file completely freezes BitComet and I have to end the program in Task Manager.
The hard drive (more specifically, a newer SSD) is just fine. It's not a large torrent, weighing in at 2.6 gigs, and I've got 16g of RAM.
As I was writing this response, BitComet decided to return to normal and I was able to excise the demon. Apparently I'm just impatient?
Is there an option, like a button or menu selection, where I can automatically delete all the old torrents? And is there an option whereby after a certain, set amount of time seeding a file, it will remove itself from the list?