|did they tell you how they arrived at that conclusion? Were they capturing and inspecting packets on your connection?|
Otherwise that is typical BS of get you off the line and don't provide any service.
can you connect a different pc/laptop to test the line? Same happening with different pc would mean its them not your unit.
Make sure you are clean.
update and run your antivirus
update and run your antispyware/malware
use disk cleanup
defrag your drive