what is fault P1611 in rover 25?

when driving my rover 25 along the road sometimes the speed just drops and there is no power! i have to stop and start the engine a few times for it to run ok or keep putting my foot on and off the excelerator for power to go through. the engine management light is on and when took it to diagnostics, it showed fault called as P1611. and they where not able to rectify it what is the reason. please help