The follow fixes may sound simple, but trust me they work !
The contacts on the microfilters have a tendency to tarnish and degrade the quality of the connection - this has no affect on the telephone service only the broadband (especially upto 8Mbs services).
This can easily be cured by unplugging the filter and plugging it back into the phone socket 3 or 4 times.
If this doesn't work, then unplug ALL other microfilters in your house - remember that every device plugged into a BT socket MUST have a microfilter - including Sky TV boxes etc.
Take all the microfilters to your modem/router and try them one at a time.
Other common problems include the use of cheap 'flat cable' style extension leads, these are usually poor quality and again reduce the broadband signal reaching your modem/router - remove it if possible. If you are using a router you can position it next to the master socket and run an Ethernet cable (known as cat5e) upto 100 metres without loss of quality.
If you have modern master socket, the lower half can be carefully removed which reveals another BT socket; this should be connected directly to the incoming BT line. Try the modem/router plugged into this if possible.
Other 'guides' have suggested that you must use the master socket - in our experience, properly installed extensions, using standard BT twisted pair cables have never been the cause of problems.
If this does not restore your broadband connection then call your ISP.
I work for fast.co.uk and deal with these problems on a daily basis.