I was having some similar issues with larger (2gb+) downloads having very inconsistent speeds, followed by hang ups.
I'm new to linux so I haven't delved into error logs but I have had some success with the following:
-- Really cutting back on the number of connections. I have 3 to my main server, with 1 connection each to my backups
-- Using a program called cpulimit, and limiting sab to 85%. This seems to stop it from spiking to 100% and locking up my pi
-- Using a version of SAB found here: http://wiki.sabnzbd.org/install-ubuntu-repo
-- Disable download while unpacking in the settings
I haven't tried changing the RAM split yet, thanks for the heads up with that one.
I'm still testing and I don't know how much of what I have done is inconsequential, but it might work for you too.
Ps. Max internet speed for me is like 500kB, with my Pi easily staying stable at 400kB. This may be what you consider slow, but it works for me.
Addendum: With some tweaking, I'm getting 480-520kb stable on a 3.5gb file, with 90% in cpulimit, 224mb RAM and 4 connections to my main server. This is maxing my connection so I cant push it any further. Now to let it run for long periods of time and see how it goes.