Low Internet Speed

I've phone ISP Afrika numerous time about the low speed that i'm getting from them. I pay for a 5 Meg line but only get between 0,54 to 0,19 Mbps for the last 2 weeks. I mean I pay for a service that I don't get for the last 2 weeks now already I phone the support people then it gets right but after 2 - 3 hours it's back to the same speed. I've been with ISP Afrika some time know and only this problem persist on happening and I spend my airtime to phone them and the problem doesn't come right. Otherwise I.m going to find another service provider that really can support my needs as I'm waiting for e-mails to come through for work and can't get them because the internet speed is up for ****.