Search the forum and you'll find a fair amount on this topic. One problem can be dirty contacts in the ignition switch and there's a thread describing how to clean them. Some people have added a solenoid relay in the circuit to increase voltage to the starter to solve this problem.
It's pretty common on the 123's. I carry a jumper wire in my TD because it does it every few months for 3 or 4 times, then clears up.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13