Originally Posted by FBinNY

In all likelihood the alloy upper body is wearing more than the Titanium bolt so I doubt a replacement bolt will help much, but it might. In the old days when Record derailleurs hung at a fixed angle, I used to wind dental floss into the gap between the derailleur and hanger to stabilize them when worn, But your derailleur needs to be free so that's not an option.

If you have access to plastic shim stock, I experiment with making shim washers of varying thickness until I found one that stabilized the RD while still allowing movement. I suggest plastic stock vs. stainless steel because it's much easier to work with.

You might first try packing the upper body with a stiff grease to see if that helps.

BTW- before you struggle to solve the play in the RD upper body, consider that your degraded shifting might be more related to chain wear. As chains age, there's wear in the area of plate overlap (not talking stretch here) which increases side to side flexibility. A more flexible chain shifts more sluggishly because the RD pulley movement doesn't translate as solidly to side movement at the sprocket, especially where the pully is farther away.

It isn't rare for a chain to develop excess lateral flex long before it's stretched to the replacement point. If you have a new chain as a basis of comparison, Flex it sideways, and measure the height of the arch from the top of the curve to straight edge across the ends 1 foot apart. Now flex the one on the bike and compare the heights of the arches. If the new one is much more flexible, consider a new chain before you work on the RD.