E320 Mystery Starting Issue, Mk. II

[abh3]

Initial starting issue resolved by replacing bad fuse in ECU box, operated as designed for bit.

New starting issue is starter begins to engage but then stops, barely turning the engine. If the key is held in the 'start' position the starter will repeatedly and quite regularly (every 3-5 sec?) try to engage again.

The new battery exhibits 1100CCA at 12.8V and jumping the connections under the relay in the ECU box causes the starter to spin the engine enthusiastically. 'Jumping' with long trucker jumper-cables from the battery to engine compartment changes nothing. It's not the battery or connections.

There are no codes and every fuse I've been able to find on the car is OK.

I've swapped the relay in the ECU box with another, same symptoms.

I feel as though I've been beset by malicious fairies, so much for a holiday weekend! I've searched and haven't found anything like this, any thoughts???