Recently I have been having an issue wherein my upload speed will decrease to next to nothing, it is always slower than than the service levels ~ 100 Down 10 Up. When it works... I will get 5-6 up but mostly the up seeds are less than 1. If I connect directly to the modem speeds are close to actual subscription levels. I tried to disable every interface in an attempt to isolate the problem but the issue remains whether I am connected wirelessly or via a hard wire Lan connection.
Can anyone shed some light on where i should begin to look?

Q1: Bandwidth
Are you sure noting else is using the Bandwidth?
Some Computer in your network using OneDrive, DropBox?

Q2: Router-Stats
During the Speed-Test,
What kind of CPU and RAM usage to you have ?

Q3:
Is it possible for you to POST your Config ?
(/export hide-sensitive file=anynameyouwish)