VW polo mk5 1.4 tdi

Hi there people my girlfriend has a polo 1.4 tdi and has a problem with boost. I you thrash it in each gear it will boost until it reaches top speed but if you are just taking it steady when u put you are foot down in 5th gear it gets to about 60mph and throws it into limp mode. But this dose not happen every time. I've already cleaned the intercooler out and the eve valve there was also a little boots pope that had been chaffing and had a pin hole in it so I've sorted that to. I bought a second hand map sensor of someone I no but still the problem consists. Any Odessa out there would be helpful. Thanks Aaron