Thank you all for your suggestions. I have already suspected engine mounts. They look good. I know this doesn't mean they are but ... if they were bad the engine would vibrate all the time when on idle I suppose. Not only every now and then.

Can the engine shocks be changed without having to raise the engine up? I don't suppose one would need to completely remove the engine to do so.

The engine is a 602 (2.5l TD), 120k miles, there are no visual defects, also all rubber connections look good, the rubber as new. I already have increased the idle with the screw on the IP, doesn't help. It appears that the shakes have no direct relation to idle RPM. Originally RPM was ~750, I increased them to 900, the shakes disappeared but only until the next drive when they re-appeared. There is no (significant) smoke from the exhaust, no nailing, starts immediately).

As for the injection pump, it's all clean. It used to have a leak from the top, between where the injector line sits on the top of the IP. It was only a minor leak but one could smell diesel inside as it accumulated under the hood. I fixed that and IP leaks no more. There are no other leaks and the pump is clean. I could have the pump removed and rebuilt ($200 for both labour and complete set of gaskets for the IP at Bosch service) but wouldn't really want to have the IP removed from the engine, one has to remove a lot more things from the engine than just IP itself to have it out.

"If every thing is fine, but you have excessive vibration at idle only -- goes away completely if you open the throttle even a little, the rack bolt needs to be adjusted. It isn't the bolt, really, it's the spring on the other end" - this describes the vibrations quite well. I don't suppose one could just unscrew the bolt and change whatever part is needed (the spring on the other end e.g.) without having to take the IP off the car?

Fred, I already did the procedure with adjusting the idle with gear engaged, braked, moved the bolt until vibrations disappeared, when pulled the throttle cable, after the engine reved down the shakes started all again. Surely enough, they disappear when the idle is at 1000 or more but I find idle of 1000 neither acceptable nor normal.

Thanks again for all response, it's well appreciated.