Sometimes when going to shift the clutch pedal goes right to the floor before I can put it in gear. So I have to lift it up and press it down again..... What causes this problem???? Slave cylinder maybe

Usually the slave will fail slowly. You will start to notice that it slowly lets out even when fully depressed while waiting for a traffic light in gear, or whatever.
Intermittent or sudden failures are usually the master.

I assume you have already checked the fluid level.
Sounds improbable, but a friend had an intermmitent clutch failure caused by low fluid level on a 124, because it has a single reservoir for brake and clutch, and the clutch fluid intake is higher than the brake intake (which is at the very bottom). It would fail on a certain grade (facing uphill?), but worked otherwise!

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1986 300E 5-Speed 240k mi.