Dell Inspiron 17 5767 - Wifi timeout during upload

I'm having an incredibly frustrating issue where if I try to send an email with an attachment or try to upload a file over my wifi connection I get a timeout error. After the timeout my Internet connection drops and I can't view any web pages without disconnecting and reconnecting the wifi connection.

This only happens when attempting to upload an attachment of pretty much any size. Prior to this and after reconnection everything else works fine eg. browsing  the web until I try to upload again. The problem does not occur when plugged into the wifi router with the lan cable and I'm now having to do this whenever I need to send emails with attachments.

I have tried un-installing the Intel wifi driver and all network driver and reinstalling plus reinstalling the latest drivers. I have also tried flashing with the latest BIOS. Basically all Dell drivers are up to date. Dells on line diagnostics dos not find any issue and neither does Microsoft diagnostics.

I also replaced my old ADSL router with a new one but the same problem persists. The problem does not occur when I use my old Dell Studio laptop or any of the other 3 family pc's in the house.

I have also tried changing some of the settings in the advanced tab of the Intel Dual band wifi adapter's device settings as referred to in one of the Dell problem solving links, including turning off the power management of the device and although this seemed to have a positive effect for a short while where I suddenly managed to upload a 6MB file, unfortunately it was short lived and the next time I tried I had the same issue.

I have absolutely no problems with downloading my emails or drivers or anything else off the web. Only uploads are a problem.

Could there possibly be an issue with the actual network or Wifi card?

I desperately need a solution to this please...

Thanks in advance 

Stuart