any ideas on my iveco 180e24 power loss?

i have a iveco 180e24 tector which on the whole pulls like a train however on occasions when it has been on a run with a load when i come to drive back it really lacks power as if turbo is not kicking in untill higher revs it normally clears but other day it did it for entire run back then next morning it was all ok any ideas please

johno
September 2011