Vauxhall Frontera lack of power under load ,

you reset the ignition all is ok until loaded up again, no pipes leaking ,replaced maf sensor,brought a new solenoid and swopped with others still same fault, but i have noticed if you get to say 70 mph with out hitting accellerater hard the light will come on , if you gently take your foot off accellerater the light goes out , push hard the light stops on until ignition reset, had codes read ,system malfunction etc,, cleaned egr valve, taken turbo a part like new , car only done 55k regular oil changes also ,,any info would be appreciated,,,,,