One failing on these cars is the crappy heater control valve on the side of the engine. Mine was very corroded internally before, so I cleaned it up before reinstalling. Now, it leaks coolant in steady small stream as I am driving. Unfortunately, the location of the valve puts it right above the distributor, so the coolant leaks onto the distributor cap. Distributor caps do not like to get wet - the car starts running rough or dies altogether. On my last trip downtown and back, I had to keep restarting it with the clutch since it would die at random intervals!
To address this, I am cutting off the valve and making one out of standard plumbing. You lose the adjustability from the cockpit, but that never worked well anyway. In fact, I have the heater core itself isolated from the system, since that was leaking coolant onto my feet in the past, and the core needs to be replaced or repaired. Since I am not driving the car in the winter, it hasn't been a priority.
See this site for a description of the fix I am doing. I'll post my own pics as well.
At least I am not alone in this - it seems the heater valve on MGA's and MGB's is a problem - or at least the modern replacment valves are a problem. My first one lasted over 50 years, so it's hard to say that it was really that terrible. The new ones apparently leak out of the box.