Tuner problem

Hi guys having a strange problem, using duo just flashed with latest vti image tuner a using usals, tuner b 28.2 only. When using tuner a (motorised) the dish moves to the correct satellite pulling in signals of 93% 95% with a good picture then after about ten mins the dish just swings round to the west as far as it can and then stays there until I move it to another sat or reboot it and then it works for another ten mins or so. I have tried reflashing the box and that didn't work. Any help appreciated. Ross