Transmission issue with a 1980 KZ250D

When I am in 4th gear under a heavy load, the transmission will sometimes kick down into 3rd by itself. Sometimes it will go back into 4th, sometimes I have to shift it myself. If I am just cruising it will not do this. It was a little unnerving at first. Unfortunately, it didn't do this when I was test riding the bike, or I would have passed.

The bike has 11K on it so I assume something is worn out. Is this an easy fix or should I just live with it?

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I will look into the clutch. It seems like it grabs a little soon. Sometimes when I shift it into first from neutral with the clutch handle pulled, the bike will die.

I changed the oil recently though, went with 10W40.

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Could it be anything else?