Gearbox Problem?

About 3 months ago, my atos just stopped. You could engage the gears (automatic) freely but the car wont move and in any gear, it just revs as though it was in neutral. My mechanic changed the oil and cleaned the strainer and this worked for the next couple of months until now. The car can be driven, but as soon as the engine gets hot, it stalls and wont move. I switch off, wait a while and it will start and move again. I had the strainer cleaner and the oil changed but this did not help this time. Please advise as to the cause of this problem and what needs to be done.

The gearbox has definitely gone. I am getting a reconditioned gearbox but without the torque converter. The supplier wants an additional amount for the torque converter. According to my mechanic, their is no serviceable parts in the torque converter to refurbish. If there was, surely, a reconditioned torque converter would come with a reconditioned gearbox. Please advise on whether the torque converter can / needs to be reconditioned or can I simply take my existing one and put onto the reconditioned gearbox?