there's a similar problem to this here

http://forum.lowyat.net/topic/443004/

but i'm unsure if the problem have been solved or not

tl;dr thread

try set your download limit only 1/4 from maximum limit u get
example.. when u not surf with ur browser u can get 200kbs.. try to set only 50kbs

i set my download only 50kbs when i want surf internet... ok what
n set unlimited when i have work to do.. normally i can get only 180kbs maximum