Hello,
As suggested, it could well be air in the fuel system, or a blocked fuel filter, but to be honest the list of possibilities are endless. these engines do tend to have a noticeable 'rock' (as does the 300TDi and the TD5), when they suddenly return to idle with no engine load. As soon as any engine load is reduced (i.e. you dip the clutch) the turbo produces much less boost and the governor inside the fuel injection pump dramatically reduces the amount of fuel that is pumped to the injectors, causing the pistons to slow down instantly (reduced RPM), hence the sudden rock. The engine then runs at the pre-set 'tickover' speed, which may have been lowered, which will create a much more dramatic rock, almost like stalling. First check to see if there is any air in the fuel. Start by slackening the small 'bleed' bolt on top of your fuel filter housing, so that it begins to weep diesel. If you see any bubbles popping up through the diesel that is escaping through the bleed bolt, it would suggest that there is air getting into the system. If you don't see any bubbles straight away, try pumping the manual lever on your lift pump. This will manually draw fuel from the fuel tank and force it into the fuel filter. If any bubbles appear, you have air in your system. This could be caused by a split in a fuel line (which wont necessarily 'bleed' fuel), or a failed seal or pipe connection. If there are no bubbles present, swap the fuel filter. Also test your lift pump properly. Remove the pipe between the lift pump and the fuel filter housing and turn the engine over. The lift pump should launch fuel at your bonnet. If it just trickles out of the lift pump, it needs replacing (at a cost of around £9).
Does the engine produce any smoke under normal driving circumstances? I'd also check your turbo hoses (two pipes). They run between the turbo outlet and the bottom of your intercooler, as well as the air inlet manifold and the top of your intercooler. If either of these pipes are split, or if the jubilee clips are slack, you will be loosing boost which can have a similar effect. If this is the case, you'll see lots of black smoke and it will feel 'numb'.
-Pos
