V70 - Loss of turbo boost, fault code

[peterww68]

Since I bought my 2002 V70 D5 6 years ago, my excellent local Volvo specialist Amber Garage in Southwick, Sussex, told me the intercooler was leaking oil. At a replacement cost of more than £500, I've never got round to having it done, especially since it's never been noticeably short of power.
Recently though, it's occasionally lost its turbo boost at low revs - instead only gaining boost at 2500 revs, and the message, "engine service required" is displayed.
My question is: Is the problem due to the leaking intercooler, a computer problem or something else altogether? I don't want to spend £500 on the intercooler to find it needs a new computer, when the car is probably only worth a few hundred quid anyway.