Why does my internet speed keep dropping?

[Sporadic]

Virgin can cap speeds, especially at peak times (evenings, usually). I know Sunday night can be a pain for me.

Also if you use too much bandwidth in a one hit (sorry, can't remember how much), you can get sin binned for a few hours, where they drop the speed for your connection to, I think, 1/3 of it's regular speed.

The green bar at the bottom shouldn't really have any peaks and troughs like yours. Personally, I'd run the test at regular intervals over the weekend, and armed with the results, get in touch with Virgin.