hi, do any of you oricles have an idea what the problem might be on my girlfriends boxster (2001 2.7 tiptronic)

in 5th gear at approx 2000 rpm it seems to surge (3-400rpm) around 40mph
it has not thrown up any lights and a snap on code reader says there are no faults (via obd)
could it be a maf? or what do you think?
low speeds seem fine and so do high, she said its getting worse?
i suppose i need to find an indipendant service centre? are there any near guildford in surrey uk?

thanks guys

 

just an update here.....
my local mechanic was chainging the header tank last week, and told me he broke the new pipe that goes from the oil filler (on the header tank) down to the engine (the pipe that you poor the oil into)
he said he "fixed it" with some rubber hose he put like a sleave inside the broken pipe....could this be the fault? does the boxster need crankcase presure? and could the engine be "erratic" if it was not right?
thank you.