If you are connecting to the Randburg BS, I can tell you that this tower is almost always overloaded. This is the default tower that I connect to, but I have since installed a directional antenna and focused it on the Hyde Park BS. Problem solved, for me.
So your problem could just be congestion. Or, more likely, it could be that you are switching between several strong BS's in your area. That's supposed to be fine, as per the technology. But in practice, it causes your signal to drop frequently, as you seems to randomly pick up ghost or distant BS's. A directional antenna focused on just one BS may be the solution.