I have this issue and I think that it is a combination of two different systems on the HDi RCZ. The first is an anti stall mechanism that senses a drop in revs and automatically applies a little throttle to bring the engine speed up, mine usually jumps to around 1200 rpm when this happens. The reason that this happens is due to the hill assist function. This causes the car to hold the brakes for .75 of a second when moving off on a hill. This works really well on my wife's EGS C4 (and on my Smart Roadster for that matter) which both have automated clutches on manual gearboxes, but can be a real pain on a proper manual car.
I notice it mostly when I park on my driveway at home. When I pull into my parking space, I stop when the parking sensors start shouting. I then just lift the clutch to creep forward until the parking sensors give a 'flatline' stop. The problem is that my driveway has a very slight slope, so the car detects this as a hill and engages the hill assist which holds the brakes. The ant-stall mechanism senses the revs dropping due to the car trying to move forward against the brakes so applies a little throttle to increase revs, just at the point that the hill assist releases the brakes. Result: an uncontrolled jump forward, a scramble for the brakes and a pounding heart.
Just my tuppence
Kevin