broadband speeds drop to 1mb,15 mins after reset??

Hi, I posted before that my Internet worked great for the first month or so, no when I go to use my wifi it speed tests at 1mb download speed if I'm lucky. Then I reset the router and test again, the speeds are back up to 25mb which is standard for my area. It's very frustrating and I've tried to ring sky, they just tell me to do the same thing over an over again. I've checked my bt line and it seems to be fine. Any advice?
Thanks in advance