So, here is the setup:
I am driving my car on the freeway in 5th gear at around 60 mph.
I slowly press the accellerator.

Here is what happens:
The car slowly accellerates up to about 63mph, then a different tone is heard and there is a vibration which sounds like it comes from the exhaust headers, then the car drops speed and rpm down to about 61mph and re-accellerates.

What the shop says:
This is normal and caused by the dual intake plenum.

What I say:
I have never felt this before and sounds like a new thing to me. I am able to reproduce for them and they say that all of these car are this way.

Does this sound right to you? Anything I should ask them to do?

Thanks for you help,
Chris