torrents are always throttled since it requires the most bandwidth, regardless of provider. what most likely happened is that the tower you connect to is more congested as more users signs up/user the internet in your area.
I suggest you maybe get a torrent server, it'll cost you about R100 odd depending on the provider but will make it able for you to download via HTTP or FTP instead, something that gets higher priority.
It is always cheaper for a fixed line though, so it really depends on how much traffic you generate as it gets to a tipping point at some stage. If fixed lines isn't an option, then I understand, but you might want to consider it if it's available in your area if the speed concerns you as wireless will always be congested and P2P traffic throttled to help ease that