Have a look at the bottom engine mount, as I have just replaced one that was damaged (rubber split on the large bush) on a KV6 and gearbox is smooth again.
Enough fluid in the box is also required for it to operate as it is supposed. See other threads for how to check as it doesn't have a dipstick or easy way to check quickly as the fluid needs to be at 40 degrees Celsius, which it is not the operating temperature when warm (it is much colder).
Rob
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 75 KV6 2.5 Sterling automatic, 3x 75 CDT (2 manual + automatic), MG ZT 190+, 620i, SD1 2300S, P6 2000SC & TVR 350i 
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