Not sure if you fixed it or not, but one thing that many owners neglected was to replace the little gas struts that help to lift the lights. When these go bad, the motor has to work harder than it was designed too, which can cause the motor itself to fail. Might explain why it has problems going up but not down.
The official servicing recommends replacing them every 40k miles, however back when I had my 480 phase, hardly anyone seemed aware of the need to replace them.
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