I tried many times, but was unsuccesful until I used the older 3.3.4 firmware and ran the firmware upload from an old Windows XP machine...

Now it's loaded on and running the 3.3.4 firmware. Pheew!!!

Unfortunately, I'm still having the original problem - this is what prompted me to try the firmware upgrade in the first place.

With the 2820Vn, my ADSL connection drops to under 1Mbps, and is veeeerrryyy slooowww... if I switch to the crappy O2 supplied router, it's up to 4.5Mbps... What gives!? I'm using o2's re-packaged Bethere for my ISP...

have you tried to load the firmware: 2820V3.3.5.1A232201 ?

I am not in the UK, dont know, if there is something special with Vigor routers an UK, but might be worth a try.

Known problem with 3.3.4.1 I am afraid, get rid of it as fast as possible!

I had the same problem of much reduced speed when 3.3.4.1 came out and I went straight back to 3.3.1 until 3.3.5.1 appeared on the draytek.com site. I switched to 3.3.5.1 and have had no problems since.
There is a beta version on uk site of 3.3.5.2 (I think) but since I'm not having any problems I'm waiting until it is fully released.

I also had the issue of the router entering a constant reboot after updating the firmware many moons ago last year.

I didn't use 3.3.4.1 so can't comment personally on the speed issue, but it was certainly an issue with some people on the forum. Even if I had, the line I had at the time could only manage 800K anyway. on o2

Returned to 3.3.0 via method you described. I had a 3.3.4 special (beta) from support which resolved the reboot after upgrade issue.

Subsequent to this updated to 3.3.5 without issue and have just loaded 3.3.5.2 without issue.

Support do recommend a fresh install however due to the number of changes in 3.3.4, so assuming you don't have too complicated a setup, save screen shots of each relevant configuration page (as well as a configuration backup for safety).