I've had this happen twice before. My 1996 740 won't start, and it won't crank. Has anyone else figured this out yet?

Here's what I know:

- The battery was strong last time I checked (recently)
- The ignition switch was changed after the first no-start, but it didn't solve the problem.
- The first two times it happened after I left the key in the ignition, this time the key was out.
- All three times it occurred after sitting only a short time (~30 minutes)
- I bought the car used a year ago, so I'm not certain of its history

I know others have also had this problem -- has anyone figured out the fix??

Thanks in advance!
1996 BMW 740iL
1992 Dodge Stealth R/T TT (modified - 400 hp)
1996 Nissan Pathfinder