If it is exclusively on 4,5,6 I would check your grounds for that bank. I had an issue with 1,2,3 a couple months ago. Turned out to be a seeping Valve Cover Gasket, was getting oil everywhere and down in the wells, but also shorted out my ground.

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05' Land Rover LR3
06' E53 X5 3.0i
07' X3 3.0si
08' X3 3.0si
17' Interceptor 3.7 AWD Sedan