intermittent excessive power (yep strange)

bit of an odd one...



some times my laguna goes like my old 90bhp one, and then some times it seems to go like at a guess it should do 120 bhp.



and then it goes like a rocket....

ive pluged in one of those iphone diagnostics things and it comes back with no faults.

(i did check that teh thing was working by unpluging egr, boost regulator, and the faults showed when unplugged)



anyone got any ideas?