I'm glad to hear that it hasn't been only me having this issue. I've been getting some really bad lag spikes a short time into my groups raids (we start at around 8pm so I figured it was likely to do with peak time traffic).

If it helps any sort of investigation my ISP is Virgin Media, I've heard that people have had some problems before with them, though thankfully mine have been relatively minimal with only a few lags spikes at worse and only having had one or two major lag spikes recently per night.