I am posting my condensed experience on this subject to spare other 103 owners from going nuts/broke or both.
I bought this PIECE at a good price with the intention of reselling it. It had a "fresh smog certificate" and drove it 300+ miles, I loved and enjoyed the smoothness and I immediatly thought what a great grandmas car(I am a die hard mercedes diesel rattling cage fool: 74 240D 4sp).
Anyways the honeymoon stopped abruptly when the car started to show the above symptoms and all other signs of being possessed, I thoght the problem was not "BASIC" tune up related because it flew thru smog.
I had the feel that the problem was electrical so I checked for spark, what a genius! no spark indeed, then I use my 54 chevy truck approach and isolated the problem to be in the rotor and cap(I ruled out the points)as I had spark from the coil but not to the SPARK PLUGS. I took the cap and rotor appart and they looked OK, I cleaned them for moisture and the mythological path to ground that will(but seldom) happens, re-installed, no luck. I went to the auto parts store and they wanted a small fortune for the parts. I was due for a field trip anyways and went to yank and pull... my luck, a fresh 89 had arrived with near new cap/rotor and spark plug wires. I yanked them and came back to fix my problem, car started and drove fine, I bought me a ginger ale and reflected on my accomplisment and got a good night's sleep... the next day it quit at a stop sign, further diagnosis revealed no spark again but now from the coil. Back to junk yard to harvest a coil and ignition module(I think that's what it is), replaced them one at a time, car started and was fine for 1.5 miles then again quit.
Now I am aging fast!, moisture on the cap again? cleaned everything, worked fine for another 3 miles. This cycle went on for a while and I chased my tail frantically. Then just for the heck of it I put my multimeter at my cigarret lighter in the short periods that the car ran and I saw voltages bouncing from 13.2 to 14.6 volts dc or was it ac?... changed VOLTAGE REGULATOR and that fixed the voltage dance. Now it started every time, would not stall anymore but idled and ran poor. I feared the worst... the voltage dance ruin the computer module!, went back to junk yard and the computer module was not there any more, #%@*!.
Called merc source and order some items for my diesel... what the heck, throw the 103 tuning guide. Pretty "BASIC" stuff in there stressing pretty "BASIC" things that I have done over and over... exept for SPARK PLUGS, I got me new ones, WOW!... car has run perfect ever since and I am enjoying it as my temporary daily driver. Now it is for sale as a good PIECE, including the extra parts and the INVALUABLE 103 tuning guide.
Don't get me wrong! I like the grandpa feel but prefer to seek it on an older diesel for a more authentic experience, so I am in the hunt for a 116 300SD for those days that may graciously come ahead.