The cooling systems of mid-engined cars are compromised by the fact that the water has to go down from the motor, then forward about a mile, then up into the radiator. Filling the car on level ground and purging all the air out is therefore just about impossible.
Elevating the rear of the car a foot or more to raise the pressure tank that much more is crucial. You'd be amazed how much air can be hiding in a 'full' cooling system. Also, bleeding the cooling system is an iterative process; you'll crack the bleeder and top off the pressure tank while the engine is running and the thermostat open, until you get a solid stream of water out of the bleeder on the radiator. Cap the system, shut the car off, let it cool overnight. The next day, you'll get a big burp of air again. It usually takes 2-3 cycles of this process to get most (not all) of the air out.
Having said all that, a car that suddenly overheats probably has another problem. If the distributor gear shear pin shears (which is usually caused by a piece of debris getting trapped in the oil pump, which jams it momentarily), the gear can spin slightly on the shaft and then catch, so the engine keeps running, but the timing is desperately retarded. So that's something to check too.
Good luck!