Hai so I got this problem that my internet is fluctuating from slow to fast and this is completely random and can happen any time of the day. This all started happening around a month ago for no apparent reason. When I run tests on sites to check my speed thay allway shows the correct speed and that there are no problem. But when for example play any online game my ping keeps going up and down from 30-500 making the game unplayable and sometimes if i'm lucky it stops and stays low, but normaly it just keeps fluctuating up and down. It's the same with sites and videos I try to load sometimes it works flawlessly and sometimes it will barely work.
If its wired then it may be a faulty DSL filter, or your best bet would be to talk to your ISP if it goes further than that. Just make sure you're calm and have the facts written down as much as you can so you can get as much done with them as possible.
Do you have steam running in the background still (when those problems occur)?
If that's in the middle of downloading something, it will absolutely destroy your ping, and going off of post history, if you still have 4Mbps internet, it doesn't take much to cause it to skyrocket