Fluctuating download speeds

[starsburnbright]

I can't seem to hit my max bandwidth. At first I thought it was down to caching issues but I haven't completely proven this yet and this could still well be the case. I get about 14-15 mb a sec at my max speed in all other speed tests but torrents range from about 5-11 usually. It might hover around 11 for 20 seconds then fall to 6 within 2-3 seconds and stay there for awhile. This is with the ports open and very good sources of seeding (1000+ in some cases). I very rarely hit my max speed (if ever) and was wondering if there's some setting that is either overpowering my connection (too many connections being open or something) that is hampering the download. My inkling is definitely swaying towards caching issues and my hardrive struggling to write at 15mb/s to 100+ 150mb rar files at the same time.