Probably two things -- insufficient charge from a bad voltage regulator or bad diodes in the allternator (bad diodes will reduce the amps delivered), and a "tired" starter.

When the diesel starters get some age on them, the windings tend to aquire too much internal resistance. This means current won't flow properly, they turn slowly, and draw an amazing amount of current due to slow rotation. Something like 2000 amps or more (noral draw under load is 950 A).

Corroded battery cables will do this too -- no current flow to or from the battery. If you have one or both battery ends replaced with clamp on ends, get new cables, the clamp-on repair ends will corrode horribly, with the result that you don't get decent contact.

Also check the cable connection to the starter -- there are three or four wires there -- the battery cable, the wire from the alternator, the main fused buss for body electrics, and sometimes another wire. Fairly easy to have to slip up into the boot when changing a starter and have it just sitting there, not under the nut.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!