when im stopped at a stop sign, my 1989 260E shakes quite a bit. i look down at the RPM's at it idles at about 400 rpm. i think thats too low. it shakes so much to where i have to throw it into neutral so it will stop. whats causing this shaking?

before doing anything expensive,
do an italian tune-up with two bottles
of techron cleaner, it might clear up
the injectors. drive long and hard.

if it doesn't work, do a regular tune up,
i.e. plugs, most especially the fuel filter.

good luck

Worn motor mounts might not account for a low idle, but they might be the cause of the shaking you're experiencing.

The passenger side mount is just below the exhaust manifold. Take a look and you may see that it's collapsed.