I have 2004 xj8 and when driving the car at about 50mph or 5th gear it starts jerking while accelerating. I let of the throttle it stops.

scan for codes

you might have to have your transmission downloaded.you have to go to a kia dealer\'put it on a machine.if the code is out it will make your trans mission jerk.cost me $400 dollars.in my 2006 kia spectra ex

best advice is dont go 40mph in 5th gear... if you are going up a hill at 40mph in 5th gear i have no doubt that it is jerking, your putting too much stress on the engine. shift up to 4th or 3rd you will be fine. 5th gear is an overdrive gear. 55+mph

sounds like the torque converter is going out...I remember on some of the xj8 and xjr models in the x350 class had these issues...the signs were a surge in the rpm cluster at about 50mph and when you attempt to pass or kick down it will ****....but as you get higher in speed it eventually smooths out...This is my insight for your issues...If it was the transmission control module it would throw a definite code.....Usually this issue with the converter was with the earlier vin models ranging in g00 to g400 numbers....and a few others

I am assuming your transmission is a 5speed manual. What is the tachometer reading when you experience this jerking in 5th gear? It sounds like the engine is being lugged. If you are trying to pull a hill or accelerate in 5th gear at less than 40 or 45mph, you need to downshift to a lower gear.

Try to perform throttle adaptation.
You will need VAG COM or similar scaner.
Better yet , take it to mechanic shop.

the best way to reset it is with a scan tool.
disconnecting the battery for one minute will also remove the code.BUT
if your smog test uses an OBD scan as part of the process,then there will be a problem,because the monitors will not have been set,as they are reset if the battery is disconnected..
most tests allow only one monitor missing,and not the catalyst .
A drive cycle will have to be done to set the monitors.
Typically cold start coolant temp less than 50F,idle for 2 minutes no air condition, accelerate at half throttle to 50mph then drive 3 minutes with a/c on at 50mph steady.decelerate to 10mph without using brakes..Accelerate to 60mph and drive for 5 minutes at steady speed without a/c
deccelerate without braking and drive for 1 minute at 40mph.
stop the car ,use braking.
Shut off the car and add 4 gallons of gas.do not shut off the car for more than 5 minutes.
drive in 3rd gear or overdrive off with rpm 3000 for 1 minute.
decelerate to 10mph.without braking
accelerate to 40 mph in 4th gear.,or overdrive on
decelerate without braking to 10mph.
test complete.
this will set the o2 sensor heater ,o2 sensor,catalyst,evap purge.and the other monitors.

i had same problem it is your cam positioning sensor

yeah it could be full of water in the gas you bought. see if you can pull a sample and check. start looking at the filters.