On my 240D a loose DV (only finger tight) was causing a misfire and I read that what happens is on the downstroke the plunger draws a small amount of air in around the DV base and the small volume of air is enough to cause the fuel in the hardline to not pressurize enough to pop the injector at idle.
Basically at speed this can be overcome by the increased volume of fuel or just the misfire is less noticeable but at idle the air in the hardline is enough to prevent the fuel from pressurizing to pop open the injector. I'd bet the same thing can happen in the 603/606 pumps if the seals are bad - that air can be sucked in and cause micro-bubbles in the fuel which prevent it from pressurizing when the DV plunger goes up.
In any event I can tell you that simply tightening the loose DV on my IP cured the miss (and the rocking at idle) and now the car is firing on all 4 cylinders all the time
