Try an experiment, try downloading a magnet link from here http://www.magnetmix.com/video.shtml & tell us what happens. What speeds do you get?

I've a horrible suspicion your problem might be due to ISP bandwidth shaping against LimeWire or p2p programs. That's often the reason for sudden drops in performance.

Also check the questions at the bottom of this post Connection problems? Check here first! (click on link) - yes that's in the FrostWire section but it has the questions easily layed out.