How old is it? If it's out of warranty you may have to look into dissasembling and relubricating the helical gears and the steering shaft. There's no settings that can remove stiffness or faulty FFB. If you can rma it go ahead and do that. It's 1 year warranty in NA and 2 years in most EU countries. Goodluck bro
There's no settings to adjust in GT Sport that has any effect on those things. You need to try and connect it to a PC and see if your issue persists before you just RMA it (if possible) Since it's a console game the thing just has to work out the box and keep working cause there's no settings for you to adjust that changes the behavior of the wheel other than FFB strenght..
If you are hard on it like FFB strenght 7-10 try turning it down to 2 for a while and see if it changes. I had 4 G29's and I know running them at high FFB load is not good for them, the motor isn't strong enough to keep it running without FFB fade or even disabling FFB to self protect. The airflow inside the casing is also horrendus so heat can't easily escape except for 4 tiny holes. A few things to keep in mind
In the options menu, available when driving, set the Torque to 4 & the Sensitivity to 10: that should take care of the stiffness. The stuttering could be due to the unrealistic arcade-style effect they added when cars understeer. It's very noticeable in road cars, but you can hardly feel it on race cars.
