[Luckypants]

The frequency ranges that can be sent down a phone line are very wide, a lot of which are inaudible and so useless for carrying speech so the ADSL technology use these frequencies to carry data. Now the frequencies close to the point where speech and data meet can interfere with each other, so the ADSL filter you fit splits these up to prevent this interference / crosstalk. Only voice frequencies should go to a phone device and only data should go to your router.

If this was occurring you would have a permanently bad connection, not intermittent as your report. I would venture that a slow download speed at certain times of the day is caused by data congestion at your exchange, rather than filter failure (leading to crosstalk). The thing to check is what speed your router says it is synch'd at when you suffer the slowdown / timeouts. It will probably still be synch'd at 22Mbps, but you just cannot get that throughput.