VW '99 losing power ?

Hi, I have a '99 vw golf 1.8 turbo. It's sort of doing the same thing it loses power when you drive and then also around town when waiting at the lights. You can hear the engine sound dropping as if it's going to die but then it comes right again. Whilst driving it loses power and shutters forward asif you are trying pull away in third gear for instance, like it does not have enough power to actually go. It's extremely intermittant and been at the machanics 4 times inthe last 45days and they cant seem to find anyting wrong with it. They say they have checed the O2 sensors, fuel line, fuel pump, intake hose, etc etc etc.... I am just so fed up with it... any ideas.