Thorguard
This may not be your problem at all, but something to check..
One possible reason is that a drive is full, or near full, even if it's not the drive you're downloading the file to. For example, sometimes I have downloads fail because my main C drive has become nearly full, even though I'm downloading to a completely different drive. If I only have 13GB free for example and I'm trying to download a 20GB file, it will fail and I think it's the site at first until I look at "This PC" (that shows all of your drives) and see one is full for some reason. If I clear some space on that full drive, it will download to that amount and stop there.. until I clear enough space on the drive that will allow it to complete download.
Why? I don't know. It's like it's caching on the wrong drive and then rebuilding itself on the other.. some glitch maybe not related to this site at all, but ends up effecting the downloads and you start thinking the site is the problem. Besides clearing enough space, resetting the computer seems to fix this whenever it happens to me as far as I remember.
When you use IDM, it likely has in the settings which drive you're caching the file segments to before rebuilding the pieces together at your chosen destination.
Another possibility is that your Wi-Fi might have some interference that causes a poor or inconsistent connection that breaks a direct download operation (IDM works for you because it makes multiple connections and also has the ability to restart where it left off on most sites and whatever other features). My computer looks better and can hide wires better, etc. with the back facing a certain way, but that also puts my Wi-Fi antennas on the opposite side as my router.. so the signal now has a lot more metal to pass through and it's also closer to my monitor, speakers, keyboard, mouse, powered USB hub, etc that could be causing more interference.. so turning the computer around so that the WiFi antennas face directly toward the router, I now get consistent full speeds and no disconnects.. or very few compared to what I was getting. So the solution if this is your issue is to turn your computer so there are less obstacles between the antennas and your router.. or get an extension for the antennas so you can face your computer the desired direction, but you can move the antennas away from causes of interference.
