Lately i've been having a problem with my internet speed on seemingly random times (mainly evenings though). My download speed drops from about 39 mbit/s to about 0,5-1 mbit/s... i've called my ISP but they've not being to useful yet ... "reset your modem- reset your router, don't test on wireless, bla bla" which are all obvious things that i've already tried. They suggest i buy a new router, but i've had this one for about 3 years and i've never had a problem with it so far.
Any clues as to what it may be / what i can do ?
I've seen someone fixing his problem by changing his sync profile in his router(?), i'm decent with computers but i'm absolutely clueless here and it's pretty annoying xD...

(can probs post a pic of my 'normal' speed tomorrow, like i said- around 39 Mbit/s)