..sounds like you have high speed but it slows down to dial-up speed. you should contact your internet provider and see what's wrong. they will help you out and describe exactly what you need to do.
There are lot of factors available to decrease your download speed like router problem,Simultaneously if you run lot of software's,There may be a chance of virus attack in your system etc..So try to check everything and all.
If you have slow download speed,Try to keep in mind the following instructions
1.Get a download accelerator
2.Turn off all unused apps while downloading
3.Don't upload information while you're downloading
4.Make sure your router is free of any interference
5.Update your network device driver
6.You must always keep your computer clean of any viruses
7.clear your temporary download history files.
8.Use latest version of any browser
9.Do regular speed test.
When i had the same problem,i followed the same ways.I am sure,it's very useful to increase your, download speed.Then you can check your new internet speed using ScanmySpeed.com