another old git

HI, I have a bike that I retrofitted with a TSDZ2 36v 250w motor assembly. It was working fine for about 6 months but it has now developed a fault. I have gone through a few forum suggestions but at the moment I am stumped. What happens at the moment, It will give power assistance for about 30 seconds now, cut in and out for a few minutes more and then packs up completely. Things I have checked, that the magnet on the rear wheel is in line with the sensor, that it is charged, I have even tried swapping out the handlebar controller from the supplied VLCD5 for a VLCD6 but none of this seems to bring it back from the dead. The first time I noticed that the motor had become unresponsive was when I stopped mid ride to take a phone call without switching the electrics off and found myself powerless afterwards. Has something burnt out/ become damaged through overheating? It's such a shame because as an old geezer this was a perfect way of getting a bit of exercise instead of getting the car out but its now let me down too many times and its not funny lugging all that extra weight around when it dies. Hoping someone can help,