Vauxhall Zafira swooshing noise accompanied by power loss

I have a Zafira B 120 1.9 CDTi (55 reg. Z19DT engine) and, between 2000 and 3000 rpm, I experience an intermittent swooshing sound accompanied by a loss of power. This happens in all gears, but is especially noticeable in 1st and 2nd, and only if I am gently accelerating through 2000 rpm. It swooshes about twice a second and the power loss matches the swooshes, so I get a kind of "kangaroo hop" type of power delivery. If I accelerate harder, it disappears. If I get to 3000 rpm, it disappears. Apart from that, it's a fabulous drive. Sometimes it doesn't "hop" between 2000 and 3000 rpm, and smoothly purrs on through. About half the time, it hops. It is also more noticeable once the engine has warmed up.

I took it to a VX dealership and their diagnostics couldn't fault it, despite experiencing the hopping themselves on a test drive. They charged me £46 for this half an hour test and then told me that a further couple of hours diagnosis would be required @ £194 inc. VAT to try to find the problem. They said it wasn't the EGR valve sticking as this wouldn't hop, just give a solid loss of power. They also said it wasn't the injector wiring loom (http://www.vectra-c.com/forum/showpo...7&postcount=17). I am at a loss and I don't want to give them any more money for them to tell me a pack of lies about what needs replacing for their own financial gain.

Has anyone out there any idea what could be causing this intermittent problem with my lovely Zaffy? TIA,

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