240D shakes at idle -- already tried the usual stuff...

[holthoff]

My 1980 240D automatic has a quick buzzy shake when idling in gear. In neutral or park, it's quite smooth (as it is when driving), but as soon as you slip it into reverse or drive, it shakes, especially when cold.

So far, I've done the following:

*diesel purge
*new fuel filters
*valve adjustment
*new motor mounts
*new idle control cable

Each of these has helped a little, but the shake is still very much there. Cranking the idle control way up does make it smoother, but I'd rather not have the car idling high all the time, if possible.

I'm planning on putting in a new transmission mount this weekend. Is that going make much of a difference? What else should I try?