Before you invest in a new one - I had a similar problem last year and didn't have a clue what was causing it, but I cleaned inside the intake and the MAF with Methylated spirit on a cloth. It made a fantastic difference. That said, I dont know if there are any reasons that I shouldn't
have done it, as I've never heard of anybody attempting to clean a MAF sensor before. Perhaps theres a reason.
Have a read of this thread which involves replacing a MAF in a V6, although it didn't turn out to be the cause of his problem in the end (In fact, Bago may have a spare one that he moight sell). Sorry its a bit long-winded.http://www.l200.org.uk/community/index.php?topic=35825.msg296495#msg296495