download stops and starts

Hi

Why is it that when I download a file it starts then after a few seconds it stops downloading the seconds start ticking off and then it resumes the downloading then stops, seconds start ticking off then the download resumes again. This can continually happen with any given file. Then sometimes, when it has done this stop start malarky it says " couldn't down load" I click resume and the download resumes and then we are into the stop start issue again. This doesn't happen with every file, perhaps 1 out of every 6 or 7.