Chain slips on low and high gear and sometimes when putting some strain into it
Both wear of the rest of the drivetrain (chain in particular can be stretched) and chain width mismatch (9 speed chains are narrower, and so are distance between cogs in the freewheel) can contribute to the skipping you see.

In general, 9 speed freewheels work neither with 8 nor with 10 speed chains.