2010 Ford Focus losing power, shuddering

so here is goes focus 12 months old and for 6 months had itimittent problem with randomly losing power while driving not completely its like the power just stops getting to it and the car starts to shudder been happening for 6 months and gagage keep saying cant find fault as no warning light coming on and diognostic machine not picking problem up.its had its oxygen sensors replaced but still happening anyone got any ideas or now anyone or yourselves who have experienced this. no warning light on and no indication of when it will happen..