FTP :: Stop Uploading Large Files

I'm facing a very strange FTP issue with one of my shared-hosting accounts, while all of my other servers are having no problems but only this one, when I try to upload a file (whatever file) larger than 500kb from my local PCs, in most cases, the file would stop uploading during the process and hang there until it times out.

There are 2 interesting things though:
The file transmission typically hangs when approximately 248kb of the file have been transferred, pls see the attached screenshot for example.

If you look at the attached screenshot, you will notice that the uploading transmission hangs when 248kb of the file have been transferred. This is very strange and what I mean is that for example, I randomly pick up a file, and attempt to upload it onto my host for 10 times, now see, 5 times it will hang when 248kb of the total size have been transferred, 3 times it will hang at other points *near* 248kb (224kb or 280kb typically), 1 time it will hang at another random point, and 1 time it might be uploaded successfully (yes, there is still a tiny chance for the file to be uploaded successfully).

My default internet uploading speed is 80kb/s-100kb/s, lately I found that, when I limit the uploading speed on my FTP client (e.g. max. 30kb/s), everything WILL WORK without any problem! No hangs, no interrupt.. Whereas when I free up the uploading speed limitation and let it upload with my regular speed, the problem appears again.

It seems to me that the FTP hangs only when the uploading speed is higher than 60kb/s. However my host provider told me that they have customers uploading without any problem at over 400kb/s, and they said "there's no problem or limitations on the server at all".

Up until now, I have done following things to troubleshoot the issue but with no luck:

Contacted my host.
Disabled/Enabled the PASV mode on my FTP client.
Tried different FTP clients on different computers (FlashFXP and Filezilla).
Rebooted my router and reseted everything with the factory default settings.
Contacted my ISP for the issue, they "did something" but nothing were helpful.
Rebooted all my PCs.
Disabled both firewalls on my PC and on the router.

Furthermore, I have asked another friend of mine in another city with another ISP to test the FTP uploading, but unfortunately he got the exact same problem. And I've done some search on the internet for hours but no one seemed to have the same problem..