For private customers (ie. Joe Average at the end) the first thing usually is congestion for such troubles (ISPs also 'shape' traffic by restricting certain services during such times), as in 99.99% of the cases you get hooked up with a lot of other people, thus you'll notice speed differences for your service due to too many users using a limited pipe now and then.
You'd need to run some speed/link/packet loss tests during those times when you have trouble with your connection to find out whats going on - there are a couple of websites and tools dedicated to this out there - but I'm not knowledgeable in that area to give advice on what and where to look for this really (personally I'd probably turn to stack-overflow, superuser section for help in that regard) as this also depends on he OS you use.
Or you got some 'funny' software on your machine at home - unless you've been using the same machine in all other cases, ie. a laptop which would rule that option out.
Anyhow, if you're on a 'flaky' line that has troubles with big downloads, the first thing you need to make sure is to use a download manager for your browser.
This will eliminate 95% of troubles in regards to 'broken' downloads.
The ones that are included in the browsers have become better over the years, but my personal experience is that a download manager still beats them for getting me large files (>100mb) as they can open several connections to the source and try to get the file bit by bit if they have to, even if the connection drops in-between - you can even restart the computer and they will keep going afterwards.
I use DownThemAll with Firefox, no idea if there is better out there. And as always, I'm not affiliated