Best way to find the problem is to logically follow the troubleshooting process until you can figure out where to narrow it down to.
In your windows command line (Command prompt), do a couple of simultaneous constant pings to 3 different, relevant IPs, like a Dota Server and perhaps another international IP just be sure it's not your ISP's routing to a specific network. Also do a constant ping to your internet router's IP to see whether it's perhaps a problem between your PC and your modem.
You can find some SA Dota server IPs here:
https://www.gametracker.com/search/dota2/ZA/
So Step 1: Open a windows command line and run this command: ping 196.38.180.20 -t
Step 2: Open another command prompt and do another ping to maybe Google's DNS servers (USA IP): ping 8.8.8.8 -t
Step 3: Open yet another command prompt and do a ping to your router, e.g: ping 192.168.1.1 -t
Leave them running in the background. It won't affect your game's network performance as the ICMP traffic from the pings are very minimal.
When you experience the issue, go look at the command prompt windows. If you see something like "destination unreachable" or "request timed out", in any of these windows, you will know if it's a network related problem. If the ping times are high on the pings you're doing to the server IPs, then it's an issue with your ADSL line or your ISP. If it's between your PC and the router, then it could be your PC's network card, your network cable or your modem.
If none of these things happen, but you still experience the issue without any of the above mentioned signs, then could still possibly be the way your ISP handles certain types of internet traffic. You can rule this out by perhaps using another internet account from a different ISP if possible.
If you however do see some of these traffic timeout problems as mentioned, you can ask Hellkom to reset your port. If that does not help, report the intermittent connectivity issues to your ISP.
If all this fails, then you will have to start looking for the problem on a software level. Unfortunately, I cannot help you too much there and you'll have to scour some forums for a resolution. I've seen some really whack issues with Dota 2 but they have mostly been caused by network related problems.