Most definitely your alt. or battery. These are very finicky when it comes to power. Unlock the cluster and you can monitor the voltage while in motion. Once you do this, before starting you can monitor the batteries at rest voltage. (POS 1)

Vid on how to unlock - BMW E38 E39 Cluster unlock and voltage check Part 1 (E46 and X5) - YouTube

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740iLDUKE
01' DINAN 7 S62 Swap - Stratus Grau
(1587 produced in SG 440)
02' 4.6is DINAN X5 S2 Supercharged
833/3258 produced | 1/27 Dinan X5's