I might have found the cause for slow speed variations in cassette drives where the speed decreases slowly and then returns to normal abruptly. I think it is the pinch roller which has either too much dirt or not enough oil in the moving parts or it is hold in position by a worn-out spring. To fix this, I must either clean the pinch roller and put a drop of sewing maching oil onto its moving parts or I need to work with fire and steel (at least some mm of it) to fix the spring.

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