This is often called the "Lawn Mower Syndrome" (you can search on that,
below and to the right for more posts). It has been reported many times.
Your plugs and oil can be fouled becuase the car basically floods. It
has happened a couple of times to my wife's 1995 855. Althought she
does move it out of the garage quitre often without the problem so it
is somewhat random. I've never had it on my 1998 S70.

The fix is to run the engine for at least 90 seconds any time it
is started to make sure it warms up a bit.