Need some more info.
-> install pingplotter and made a test against for example. google.com. Choose sampling "Unlimited", trace interval "1 second" and samples to include "ALL". Take a screenshot and post it.
This will show if you have any leaks.
If you have a router or something like it, remove it and just shove the cable into your comp and try. That will eliminate the option of it being your router/etc.