Engine management light !!

Hi, I own a vauxhall vectra S reg Automatic... and recently had the engine management light keep coming on. I took it to vauxhall and they put it on there machine and it showed up nothing at all , So now im thinking what the problem could be . It was ok for 2 weeks and drove ok then out the blue on came the light again , you can still drive the car but it sems as though it is taking longer to change gears and when I was at a stop last week the engine cut out on me completely. This has been happening for 4 weeks now and comes and goes and I am still at a loss as to what it might be , I have been told that the crank shaft sensor could be the problem and make this happen but im not completely convinced. Any help would be appreciated many thanks Clive..