electronic management system on vauxhall corsa 1.2?

the EMS dashboard light keeps coming on for longer and longer periods.this is most noticeable when the car is under load i.e. when going uphill or accelerating with 4 people in.i have to go down the gears to keep up a reasonable speed.i've checked all the fluid levels and they are now ok.spark plugs are ok, not sooty or oiled up,gap is ok. any ideas would be appreciated. thanks in anticipation.