Absolutely BAD service

I upgraded Okt. last year. My subscription was R488.00. That included 200min anytime, 100sms, 300mb data and an addional 300mb add on reaccuring with 165mb promotional speacial. ( in total i got 765mb of data) for R488.99. per month. For 5 - 6 months everything was fine. No problems! Then my bill got higher and I phoned. They said my bill is correct. I denied that and phoned and phoned etc again & again. I got reference no etc. Now my subcription is much higher and i get only 300mb data. Every time i try to sort it out they either say nobody has started to investigate they will esculate the matter. Head office refuse to put me through to somebody to lay a complaint. I have struggled since April with this. The mtn store said they cant help, i must stop paying my contract and go to court...... how can they increase my subscription by almost R150.00 per month and not give me all the data?? I cant argue with them as they will then just ignore my requests. What can i do? Mtn is in breach of our contract by increasing subscription and giving what I sigjned for.