1985 745t

I've had this problem for a little while and can't seem to pin down what itmight be. No info in the FAQ seems to point to it, so I figured I'd post and see if anyone had an idea.

Whenever I start my brick cold it kind of rumbles and sputters as it gets going. It doesn't stall, but it seems to stumble a bit like its having trouble getting going. Step by step:

1) Turn the ignition and start the car
2) Tach up slightly then down to around 300 or so and climbs as the engine gets going ocer the course of 5 seconds or so
3) Tach settles around 850rpms or so and conitinues to sound a little rough until it gets warmed up, at which point all the problems seem settle out.

Anyone know what this might be? I should also note that it isn't generally apparent if I have driven the car within the last 6 hours or so, even if it is cold. The problem is most apparent when the brick has sat for 1-2 days without being run.

Any help is greatly appreciated... tired of my brick sounding like it has a lung infection...

thanks in advance,
rt