Disappearing Data

I am loading 1 Gig of data a day and just checking my e-mails and replying to them, (approximately 10 to 15 minutes) and I lose 500mb of data at a time - for just being connected a couple of minutes at a time and not even browsing the internet?. I have tried to phone MTN's customer care and is being referred round and round in circles. I spent hours on the phone and only once I spoke to a consultant who gave me another number to phone and this number seems to be incorrect. I have asked advice at a branch and was given a number to dial which also appears to be wrong. I cannot get an answer!!! In the meantime it is costing me a fortune. If it is a wrong setting, I can fix it but I cannot get hold of anyone to help me. This is ridiculous. I have been using the same sim card and same ****le for years and this has never happened before.