Distance also plays a factor in WIFI strength/speed (I think). The farther you are from the router, the lower the WIFI strength, the lower the speed.
As for torrent speed, amount of seeders (people who already have some or all of files you have downloaded downloaded) also plays a factor. If you have 1 people seeding, naturally you get a low download speed.
If you don't want that many speed fluctuations then go for Petarded's advice.