92 190E starts fine, idles fine, but stalls at 1500RPM
 

My 1992 190E 2.6 has been performing perfectly until several days ago. Wife called me and said the car had suddenly stopped running while she was on the way to work. No lights came on prior to the breakdown and all gauges read normal. Car will start fine, idle smoothly all day long, but apply the gas and it "coughs" once and stalls at exactly 1500 RPM. During the idling, everything appears to be absolutely normal as far as dashboard instruments are indicating. The car does this cold or hot. I've got all the idle I want between 900 RPM and 1500 RPM but nothing else. Wife later remarked to me that she drove through a moderately deep puddle of water on the way to work, but I've checked everything that I could and it appears to be dry as a bone. She also did not break down until about ten more miles into the drive past the puddle, so I don't think water has anything to do with the problem. Have read through threads but don't see anything that matches this problem, and have noted recommendations on temp sensor, fuel pump relay, and O-2 sensor. I spent the better part of two days eyeballing what I could on the car (I didn't disassemble anything for inspection except to remove the air cleaner cover and check for water...none was found), and am out of airspeed and out of ideas on it. Any suggestions? Any help would be sincerely appreciated!

There are only two things I know of that would cause these symptoms, one is the fuel pump(s) not working properly and the other is the ignition coil. I have seen the coils fail where only under a load.