MTN data depleted after 5 min!

My husband data was loaded on 17th, he didn't use the modem as he is working for a company were he worked 12 hours, so the modem is locked away in the house. Two days after his 1 Gig was loaded he went on the internet, he was connected for about 5 min. then he receive message to say his data is depleted! how is it possible? One Gig in 5 min, and that on a side where he always go with his Vodacom data and then he is longer on that side with less data. All our data that we received from the contract don't last long, but if we buy data it last longer and we can go on the internet for longer periods of time. What is Mtn doing with our data? I'm very disappointed with Mtn. my first sim was Mtn and I always stay with Mtn, but if they keep on doing what they are doing, then I will not renew my contract. Mtn was always in my eyes the best, but now I'm not so sure any more. It seems that it is better to go to Vodacom, my sister don't have the same problem as I do and my husband also with his private phone do not have the problem we receive with our Mtn contracts data.