saab 93 - 1999 diesel looses power

[polotop]

 hi,
I have a friend who is a former mechanic in PORTUGAL but was use to working with older cars.
He has a saab 93 - 1999 diesel that when it reaches 140 kls it looses some of it's power.  He has given the car to some former saab mechanics and they have exchanged some parts. There is some improvement but not fixed completely. this slow in power also occurs at slower speed.
Any suggestions.
thanks for your help.