My 87 300D has slow engagement between neutral and drive too (about 1-2 seconds) and occasionally I have a slip if I don't wait long enough. Other than that the transmission behaves perfectly. Could it still be the spring?

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2004 VW Jetta TDI (manual)

Past MB's: '96 E300D, '83 240D, '82 300D, '87 300D, '87 420SEL