The starter is acting up. Wind very slowly on ignition. It was taken off and apparently 'serviced', but the symptom persists. At times it just seizes altogether. The mechanics at times take to tapping it firmly with any tool at hand, and then it obliges...until the next seizure episode. Quite embarrassing when it stalls.
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I know this is stating the obvious but the battery is okay isn't it?
Try connecting a battery jumper lead from the battery positive to the king lead terminal on the starter.
If it then cranks normally then it's likely to be an issue with the king lead and/or the terminals.
If it still cranks slowly or not at all then it's an internal problem with the starter.
Bad/dirty/corroded internal connections, seized bearings, or worn bushes (or two or all of these).
If it's just been serviced (and they've done more than just spray painting the outside of the starter black), they've missed something.