Help! Internet goes from fast to slower than dialup speed!!?

I have ADSL2+ broadband with Netspace which I have had for over a year now. For the past week the internet speeds have gone from fast to extremely painfully slow. I have run many speed tests which sometimes show the speeds above 1mb/sec but then will drop and sometimes not be able to go over 1.8kb/sec!! I have been on the phone to my ISP three times now (for a long time each time) and they have reset the connection from their side but it has not helped. When I turn the modem off and back on again the speeds are high but immediately start to drop again. I have tried changing back to an old modem to see if it was a problem with the modem but it did the same thing. The modem is only about 6 months old anyway. I have run virus scans, disk clean ups, defragmented my system and cleared all internet history etc but nothing has fixed it. The computer itself is running fine, it is just the internet. Any ideas on what the problem could be?? I really appreciate any advice as obviously this is very frustrating!! Thanks to all who reply!