Going into limp mode/turbo not kicking in. engine management light comes on.

engine management light coming on after driving down the road for a while and turbo wont kick in causing vehicle to not accelerate over 30mph. after stopping and turning off engine for a while things come back to normal you can drive normal again then it builds up and light comes back on and problems start over again. i have cleaned out the engine with decoking spray and just about to clean out the egr valve does anybody have any suggestions.