V70 2004 Belching smoke
 

My 2004 V70 2.4D d5 has started to belch smoke at low speeds, whilst having low power, belches black smoke when kicking down to acelerate can be slow pulling away.
However, this can be intermittent and car will zip away from junctions and accelerate nicely at other times.
On the motorway initially smokey then clears and will drive normally for hundreds of miles.
Fuel consumption is just under 44 miles per gallon of Diesel.
Local garage mechanic says it looks like the intercooler is punctured.
I'm not sure how or why he says that.

Any ideas out there??