What steps did you take already with your ISP to try and resolve this?
Can you send them to yourself?
Anything in common with the clients you send the files to, might their mailboxes be full?
Give this a try:
Download and install FireFox.
Try uploading with these sites:
(Up to 2GB per file - no file limitation - no registration)
https://www.wetransfer.com/
(Up to 200MB per file - no registration)
Jumbomail | sending big files | transfer large files | send big email
Are you using the Dropbox software or the Dropbox site?
What is the file size that you are trying to upload?
I'm assuming you tried the obvious things like to reset your modem (usually with a reset button on the back), unplug modem from power and re-power, restart computer?
For how long was this working for you and when did it stop working? Did you try to restore your system to a date is was working?
I'm researching as you answer.
Right click "My Computer", Goto properties\Device Manager\Network Adaptors. Double click on your adapter and click on "Advanced" tab. In the property window, look for "Speed and Duplex". Click on it and tell me what it says in the Value box on the right.
Also, what is your Internet speed provided by your ISP?
Also, do you connect via telephone company or cable or is it just the satellite?
Anything in the Event Viewer that might give some indication of a problem, probably a warning or info entry?
If you ISP drops you to 56K, could prior uploads be stuck in the system? If there was a backlog somewhere, ISP or your machine, it might slow things down.
You might try running a tracert to one of your upload locations and see if any large delays are showing up.
Could the source of the files be a problem?
Do you have an anti-virus that checks the files as you upload them? I know some can check inbound and outbound traffic. I don't see it in MSE, but just in case you have something else running.
I would probably try uninstalling and reinstalling my network adapter.
I also like to open Task Manager and watch the Networking tab during transfers to see what speeds are being used.
Ok great. Click on the drop-down box and tell me if you see your speed there. If you do, change it to your speed, restart the modem and computer and try. If not, tell me what speeds are there.
Choose 100 Mbit/s full duplex, restart modem and computer. (The restarts are just to make sure).
duplex mismatch can cause speed issues. You can read about it here:
Duplex mismatch - Wikipedia, the free encyclopedia