Sometimes the "non-scientific" answer is the best one.

I too own one like yours, and my wife rarely drives anyway, so I try to take it out once in a while. I don't have the three-second start issue like yours, but the valve ticking does occasionally occur and then it goes away after 10 or so minutes of driving. The car sitting for long periods does attribute to this in a way. Different oil viscocities can improve it somewhat, but it will still happen from time to time.

I think the three-second start may be a different issue, like some sort of slight fuel pressure leakage from sitting for long periods of time. So on startup, it takes a little time for the system to develop pressure. The accumulator on my W124 performs this function, but I can't comment on the R129...after all, it has proven to be so reliable, that I haven't had to root around the engine compartment at all!

Try to take the car out a bit more (weather permitting). These cars are meant to be driven, and sitting for long periods of time will eventually get to the miniscule seals and such throughout the vehicle. A woman I know well has had her second coolant pump replaced on her 93 R129, and the car only has 8K miles!

Also, "real" issues that require attention may take several months to manifest on a rarely-driven vehicle, versus immediately on a daily driver...

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car