Here's a test you can perform to see if the bonding strap to the engine is a good connction. I know you cleaned this bonding but you may want to try the test anyway, just to say you did.
96 + XJ Body Ground Connection Test
This test checks the condition of the vehicle body ground connection (Braided cable on firewall to engine cylinder head bolt). This test should be performed with the battery positive cable removed from the battery.
Start: Disconnect both battery cables, the negative cable first, then the positive battery cable. Reconnect the battery negative cable and perform the test as follows:
(1) Connect one ohmmeter test lead to the vehicle fender. Connect the other test lead to the battery negative post.
(2) The resistance should be less than one (1.0) ohm.
(3) If the resistance is more than one ohm, check the braided ground strap connected to the engine cylinder head and the vehicle body for being loose, corroded, or damaged. Repair or replace the ground strap connection and replace bolt at firewall connection or nut at engine cylinder head bolt connection, if required.
Finish: Disconnect the negative battery cable. Reconnect positive battery cable, and then connect the negative battery cable.