I'm having a similar problem and I've been trying to track it down and it may be a coincidence but I replaced my timing chain, Vanos, rebuilt injectors, etc., as well. I plan on installing this kit (check valve kit 16149068988) to see if it cures the problem if I can ever find a vendor that has it in stock!

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01' 4.4 Sport - Dec 2000 build
01.5' - Audi B5 S4
97' - FZJ 80 Land Cruiser