Upgrade Gone Wrong!

I have upgraded to a 40 GB data contract in April/May and ever since I have been experiencing very bad service, I can hardly stream 5 min podcast/song/video without buffering till it requires me to reconnect again. I made a call to MTN and the consultant advised that they will reboot on the system and that I should also switch off my phone and on again, it should be sorted but everything is still the same yet I am paying every month for this bad service.