Could be just contention during busy periods. Who's your ISP?
When you notice it's slow check the ping times or do a tracert to your ISP from your laptop. Also go to
www.adslguide.org and run a speed test.
Then disable the wireless network at your PC and run the same tests from there. If you get a comparable slow result it isn't your wireless that's compromised and the problem will lie between your modem amd your ISP's pipes.
Traceroute will give you the url of the server that has the slowest hop and hence identify the bottleneck.