No power, engin light is on,

Occasionally, it occurs to me that my Ibiza loses power and jerks when driving. I've found out that, if I leave the key one more second when starting the car, these failures do not take place. However… 

Car seems to loose power going up steep hills and changing down gear doesn't seem to help other wise car runs fine.

My car loses strenght occasionally and it is not even able to move along an uphill road. The engine has been serviced and spark plugs have been replaced; injectors and the fuel pump have been cleaned… 

When driving Around the engine light comes ON and loss of power. I stop the car, put off the engine for about 30 seconds and restart the engine. Engine Light goes Off and power is OK. What could be th… 

When driving the engine light comes on and loss of engine power. I stop and put off engine for a few seconds and restart the engine. All goes well and vehicle moves OK. What could be the cause.