If it were me I would set my UL slots at 3 and bandwidth at 10 or 12 and I would disable UP you do not have allot of UL bandwidth to spare.
I have my max # of DL set at 50. I do not know that this makes any difference it is just where I run and it works for me.
My educated guess, ( the reason I say educated I have done allot of testing with this) seeing speeds of 120 UL 11 DL when running your UL at max and trying to DL at max you are probably creating a bottle neck. Thus when you are DL LW needs to communicate with the host and let them know what packets you have received. And if you are UL at max it has to wait in line to send it's message to your host. I like to have at least 5kb of spare UL bandwidth if I am DL heavily.
I believe that my explanation is correct and if it is not someone will correct it for me.
I do not know if you are running XP or not but XP default settings are not very efficient. The tool below will allow you to adjust your MTU setting and also has some better pr set settings. It is freeware.
http://www.cablenut.com/
There is also a very good forum at the cablenut site.
http://www.broadbandnuts.com/index.php?board=12