i t does not happen too often ,we live in Colorado....but it seems totally related to the moisture in the air...any ideas?...blessings,Kris

My old '81 used to do this a long long time ago. It was the ignition wires. For while there I had to replace the wires every year or two...and then it subsided. Maybe the quality of wires improved by the late '80s.

Look for moisture under the distributor cap.
But..

My best bet is:
If the crank position sensor on top of the bell housing has not been replaced, it is time to do that NOW. The insulation on the wire to that sensor degrades over time and shorts to the engine block or between wires. This is one of the few things that will cause a '91 to leave you stranded. I can imagine that you could have a wire that gets worse when damp. This is an inexpensive fix that is easy to do on a stick shift. From posts by others, it sounds like it may be a little tight to get to on an automatic. Don't break the clip and don't drop the bolt through the hole in the bell housing when the sensor is out!

Most of the time, it's because of excessive dirt (and perhaps salt spray, which is highly conductive when moist) in the engine compartment.

To test this, jus take a look under the hood when it's dark, and have someone try starting it. Chances are, you'll see a spectacular laser show! The dirt, when moist, become conductive and drains away the voltage from the secondary (high tension) wires, so there's insufficient spark to start the engine.

See, there was a reason why your dad always told you to keep your engine compartment clean :-).

Well, I said "most of the time..." above. The remainder of instances is most likely that your spark plug wires are probably just too old, and they're cracked and (like with the dirt) just letting voltage get away through other paths. If a thorough cleaning or washing (and then thorough drying, before trying to start it) doesn't help, your next best thing is to change the wires.

Mine did this recently, it was the rotor.
--
1985 240 DL, 1989 245 DL

To add to David's thoughts, a fairly well-known test around here is the "midnight" test. Preferably when the problem is occurring (but it may be helpful anytime) get in as dark a conditon as you can (probably in a garage), and watch under the hood when attempting to start. If you do have problems with your spark plug wires, etc., you should see sparks flying around in the dark. If all your high voltage items are sound, this should not happen.

--
David Armstrong - '86 240(350k km?), '93 940T(270k km), '89 240(parts source for others) near Toronto

You might also check the high tension wires, both distributor-to-spark plugs and coil-to-distributor, as well as the distributor cap. When elderly, microscopic cracks develop in the cap and the insulation. In damp weather, these cracks get moist and more conductive; the high voltage produced by the coil can then leak to ground.
--
David Brick, Santa Cruz CA, 1988 245

Did you check the fuse that is under the hood on the driver side against the fender not too far from the battery, small 25 amp fuse. You may want to put a fresh one in there and once in a while pull it in and out to remove any corosion.
--
'89 244DL M47 164K miles

That fuse is no longer there after '89, I think... definitely gone by '91.