240D starter sometimes won't engage

[ak_pdx]

Hello all, I've been away for a while...

This is a 1977 240D (manual conv) that I've been dealing with for the last 5-6 yrs.
Last year first the alternator failed (replaced w/used one from my "spare" engine), and shortly after the starter failed. Also replaced with a used one (also from the engine I used previously, but that was sitting outside for 1-2 years)

The starter on the first try just spinned fast, on the 2nd started the car. This would happen very frequently, 2-3 or more free spins before engaging, sometimes even started spinning after a few secs of cranking!

Replaced starter with an 'unknown' used one; same behavior.

Decided it's a bad solenoid, bought a German-made new one ($59), installed in the starter from the prev engine. Still spins often, more in cold weather.

Tried to observe voltage at the + cable at the solenoid, no obvious drop when starter goes from cranking to spinning.

Could I have bought a faulty solenoid?

Thanks!