Hi ,if enyone can help ?

I had bt broadband for 1 year download 120 kB/s,upload 50 kB/s one moths ago I switch to bt total broadband with hub.
After switching wait for one week to optimalization period finish and get same speed.
However after 14 days is my download speed less than 50kB/s and upload is about 50 kB/s.
In Hub DSL conection details I had downstreem 1054 kbit/s ,upstreem 500 kbit/s.
If I downloading something from internet It's start at 180 kB/s but keeps droping until is about 50 kB/s

Keep in touch with BT ,every time they saying something diffret and last information from them is
that  my PC causing it.
I format HDD and reinstal everything but it's still same ,try conection to hub by wrilles or ethernet.

If enyone can help if is really my pc or bt support try blame only my pc ?
Thank's a lot