Crappy Internet performance

I am paying for the 300mb service from cox and I never get anything resembling this speed on a speed test.  Every few months my service slows to a crawl and I have to have cox come out and tinker with filters etc.  Not sure why this is such an issue but I am thinking refunds are in order.  Tried all of the usual stuff.  Modem resets, disconnecting my modem from all of my internal equipment (router, switches, etc.) and still have terrible results.  Getting about 30mb/sec right now after 2 modem resets.  Anyone else having ongoing repeated issues like this?  Very frustrating.  To the point that if it does not get better I will switch as soon as a viable competitor exists in my area.  Definitely going to drop my connection down since I have yet to get anything near what I am paying for.  Ridiculous!