no power in peugot 307 diesel?

my 05 reg peugot 307 diesel estate will suddenly lose power.The engine won't rev to more than 3000rpm.If I stop the car for 15 - 20 seconds then restart the car revs normally.I am not getting any warning lights or codes.this fault normally happens when driven fairly hard but has happened when coming off a motorway sliproad at about 40 mph.Has anyone any idea what causes this, & can it be rectified?