Does the shaking stop after it warms up to operating temp?
Could just be surging from the cold air being drawn into the engine along with the engine compensating for the cold weather by adding more fuel for a cold start.
My idle is pretty ragged on my '92 535i when it starts up. Smoothes out after 30 seconds of idling. Please note that I just had the entire top of the engine rebuilt. (Cam, rockers, headgasket, distributor and rotor.)
If your car REALLY misses all the time (like mine used to) I would suggest looking at your ditributor. (That's the part that all of the spark plug wires come out, of and go to, your spark plugs.)
Mine was so cracked it was jumping to a grounded screwdriver placed near it! HAHAHAHH! God I hate buying used.