After carefully reading you first post it sounds like an ignition coil , or whats called a spark plug connector is failing, when that happens the ecu shuts down the misfiring cylinders so your 6 is now a four cylinder, take it to a shop so they can run an engine smoothmess test and if its acting up it will show the dead,s cylinders, ive had my 94 e320 for 18 years and its very reliable and easy to dignose, lets us know what fixes it, a bad coil will cause shaking and with 2 cylinders down will be slow, you have 3 coils and 3 connectors