Could be both, valve cover gasket and the lever...i had the same problem (like a few of us) i did the Valve and new plug 2 weeks ago, clear improvement, no more shaking and misfire. Yesterday i've fired her up (still cold here -15) idle was unstable.... I've let her ran at RPM 1500 for a couple of minutes ....SES came on ....the lever repairs is costly, turn in off and re-start is the way to go i guess.

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2006 X5 4.4i premium, Sport package ,Winter Package, Sat Nav. AFE Power stage II Production : 08-2006
2012 BMW 128I Convertible Black
2011 Mercedes Benz B200 sold