Lack of power. Like really sloooow.

[Tom]

Hi guys, My uncle has a 307 estate 2.0hdi not sure if its 90 or 110bhp. Now i havent drove it, but he says theres a huge lack of power, but the catch is its only occasionally. Some times its absolutely fine and drives as it should. But other times he says its dead slow, like 3rd gear foot flat cant over take a lorry slow. So im going to have a look over the weekend and see what i can make of it just wanted some advice before i go into it.
Im thinking perhaps an issue with the maf? Or some other sensor or some dodgy wiring somewhere? Thanks everyone