You seem to have multiple problems.

First, if the md5 checksum is bad, then don't do the burn, the ISO file is bad. How you are getting bad and different md5 checksums I don't know, particularly with the torrent. I don't have enough to go on here, but with the very limited information that I have i would guess that you might have a memory problem, but that is only a guess. If you don't yet have an ISO that passes the md5 test I would suggest downloading on a different system. Also, download memtest86 and run it on the system in question.

But you also talk about errors during the burning verification phase. I hope that means that you have an ISO that passes the md5 test, otherwise burning would be complete folly. If you have a good ISO then you can stop trying to download and should focus on getting a good burn. The usual suspects in a bad burn that fails verification are high speed burns, bad media (even name brand media, don't get me started on Sony), and a bad burner. Since you say you have read the documentation, and I hope you mean the downloading faq, I'll guess that you have considered all of these 3 problems already and I'll suggest two more things that could cause problems, using 650 meg media rather than 700 meg media to burn the CD is the first. Don't be offended by this, I doubt that you did this but I really should mention it because it has happened here in the past. The other thing that I can think of that might cause verification errors is a failing system, again memory a likely failure point, although it could also be a power supply issue or even a CPU issue. Without knowing anything about your particular system I would suspect the power supply first, but with the downloading problems my mind comes back to memory. Again a good approach would be to see if you can do a low speed burn on a different computer with better results.

That's about all that I can offer now, but please post back and provide more information.