Torque Converter
 

(This is for my 2005 XG350 with 114,000 miles.) I have noticed that when on the highway it will 'shudder' when it reaches the highest gear. I think I've narrowed this down to something called Torque Converter Lockup Shudder. For example: after the car climbs a hill, and has downshifted, it will shift back up to high gear and shudder.
I called the place I normally take it to for oil changes and they said that I shouldn't even bother with a transmission flush and fluid change with that many miles. I didn't even tell them how it was acting. I am not sure I really believe that flushing the transmission is the way to go anyway.
Do you think that a fresh fluid and filter would correct my issues? I really don't know when the last time it was changed. I've had the car for 24,000 miles and I haven't done so.
Thanks,Scott

The trans fluid should be changed every 30,000 miles. No, I don't think a fluid change now will help but I wouldn't expect the trans to last much longer either. I'd suggest start saving up for a new car or at least a replacement transmission.

I wonder if I had changed it right when I bought it if that would have made a difference?

Possibly. Usually, when the trans fluid isn't changed for years/miles it's better to just leave the fluid in there. It won't last forever but it will last longer than replacing it. What happens, the internal clutch backs wear and the worn particles get in the fluid. That then helps the clutch packs work. When you change the fluid, all the clutch particles are removed and the trans starts slipping.

Would you use any additives such as Trans X?

Not personally, no.

Hey, this problem has become non-existent now. Could it be affected or rather aggravated by the heat and so it's not so bad in the cold weather?