car 1992 XJ6 3.2l

I know these cars have a slightly harsher gear change then the 4.0l but has anyone had the following issue and know a solution or is it just the gearbox itslf

done atf/filter change soon as got car..

ushifts are harsher then should be but gears are not slipping, downshifts are smooth as silk

been told might be Throttle position sensor, can this be adjusted / tested manual or would it require a specialist i've heard it needs Jaguar PDU diagnostics computer to adjust, is this only at the dealers

 

My guess would be the torque converter is beginning to lock up.

 

I have an early 88 and it shifts funny sometimes and sometimes its very smooth and will only use 3 of the 4 gears. And then sometimes it will use all 4 gears but chatter when shifting to 3rd and 4th if it uses all gears. Almost fills like a slip but like a grab slip,I changed the fluid but not the filter yet and it got better. Since you cant drain the converter I am draining the fluid twice before i drop the pain and change the filter and fluid that way i get all or most of the old fuild out. I drain the fluid and drive it for a week,drain again drive a week and then drop the pain clean and change filter this should get most of it out. The old stuff look like oil but didnt smell burnt but it needed changing bad.