internet performance issues

[pSychicrAlly]

hello guys i've a 10mb internet connection from Videotron here in Gatineau, QC. i have a voip service from vonage as well, so my internet setup = isp modem --->voip router ---> PC. my connection has been in the past rock solid. but for the last 3/4 days the performance has been all over the place. go give an example, running the same speed test over and over again i can get a throughput of 6.7mb to 57 kall within the span of 2 seconds. and this can be verified by the way webpages load, one second they'll load up immediately the next they will take 5 mins. i have spoken to my isp and as far as they can tell there has been no degradation of my signal over the past few days. and yes i know cable connections, beings shared by nature, can cause pockets of slowdown, but a 10mbit connection wouldn't become instable on a second by second basis beacause of that i would think (even uTorrent refuses to connect at any times). now the only thing i can think abt is it might be something wrong with windows. but as i work crazy hrs, i am kind of hesistant to try and take the time out for a fresh installation. i already tried doing a system restrore but so far i have been unable to restore the system to an earlier date.

do you guys think a problem with windows, or even a possibley undetected spyware/malware (i run microsofts Windows defende and it reported a problem 2 days ago, but the problem persists even after it removed the threat/problem) could be causing these horrible lags ans drops?

please dear anadtechies! i need your advice!!!!!

p.s. the issue is the same without the voip router acting as the default gateway.