2000 Vauxhall Corsa engine shudders

hi when i get up to between 30-40 mph and change into fourth gear the enigine begins to shudder and really struggles to keep the speed or gain speed as this happens the enigine malfuction light flicks on and stays on for few seconds and goes off i know i am not changing gear too soon and that there is a fault could you please help on what it could be and the price i would be looking at thanks charlie