Sounds like this is what's known on brickboard.com as the dreaded "lawnmower syndrome". No known cause or cure. I've had it happen many times on my '95 850. Here is the sequence of events, you'd be well advised to avoid them.

You need to move the car some morning to get your mower out of the garage. So you start it, back it up 30 feet, and shut it down. Later in the day you go to put it back or to run an errand. It cranks and cranks but does not fire up. I've cranked it for 2 minutes straight before getting it to catch. When it starts, it will sputter, belche, stumble, then stall. If you get it started, I've found you have to really work the gas pedal aggressively to keep it going. The best thing is to get it in gear and driving ASAP. Or you just let it sit overnight and it usually clears itself.

Therefore, whenever I have to move that car, I ALWAYS drive it around the block once. I find that if I don't, the problem will ALWAYS occur, regardless of the weather or season. It is very weird, but well documented on Brickboard. I have never seen anyone with a plausible explanation. Many have been to the dealer about it, including me, but there has never been a cure determined.

So we just live with it.
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'94 945t; '95 854 base