ford focus wants to stall?

i have a 2000 1.4cc focus zetec, when i come to traffic lights or give ways the battery light will come on for a second or two and the car will try and stall. if i hit the accelerartor and bring up the revs it will come back down to idling speed and will continue to idle perfectly. i have noticed some mayonaise on an air flow pipe coming from the top of the engine block to the air flow box, i have noticed the air filter had a slight patch of oil on it, i have since cleaned the pipe and the problem is still ongoiong

ps if the car does stall its slightly difficult to start