Volvo V70 2.4 D5 bad starter motor

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A few years ago the starter started behaving like this intermittently, sometimes it would even disengage completely. I followed SiRobb's 'how to fix a starter motor' guide, after which it lasted for about a year before the problem appeared again. I suspect it's the solenoid that's actually bad. I just bought a used starter for €50, which still works fine to this day.
SiRobb's guide:
Part 1:    • How To Fix a Starter Motor Part 1 Taking A...  
Part 2:    • How To Fix a Starter Motor Part 2 Reassembly