Jaguar xj8 2004 (x350) when driving at slow speed (50 -80 Kmh) the gearbox undecided which gears to use. Rev counter goes slightly faster and slower. Problem goes away when driving faster. Also lumpy when drive, kicks on pull away and just before stop.

You could have something else wrong such as abs sensor this will put the gearbox in limp mode you will need to get the car diagnosed with computor

it is slipping; you will need to rebuild or replace the transmission.

when vw started to use the computer controlled gearbox system a criticism was the shift was very harsh,to counter this they made it rev slightly before changing.so that you know the system, try this out....without touching the gas.....put it in d and let the car accelerate to its max speed,stop,then use 1/3rd of gas (keep your foot still)and let it do the same...stop,then,if safe,put your foot to the floor,hard, notice when the car changes gear(revs/speed)then if you drive,only using the first1/3rd of the gas,the cars change pattern will alter to suit.......wierd it is.....

In the engine compartment, you will need to locate the transmission dipstick. Usually on the left side of the motor, labeled "Trans fluid". Pull it out and check the fluid level. Your problem could be as simple as putting transmission fluid in it. That will make it slip. You fill the transmission right down the dipstick tube.

You have a faulty ECU which will need replacement. You may have damage to your gearbox now as a result.

lots of old cars were + earth but not all .dont think this will make any difference ,is this rev counter for a 6 pot or four?? check terminals on back of unit??does one look as though it the earth ?? if not then one goes to earth and the other goes to the points side of the coil.easy

When looking for a "faster" transmission you need to check the actual individual gear ratios of each gearbox and match anticipated rpm to available peak torque of the engine. This gets a bit complicated but likely if you speak with anyone who drag races a similar vehicle they can hook you up with charts & formulas. As far as top speed goes, that's all determined by your engine and differential/tire combination as all units are 1:1 in fourth gear (fifth gear is overdrive and may give you slightly more top end) For a street car, there are far easier ways of wringing out a bit more performance.

dont know for sure on jags but normally id be checking out the clutch

its either low trans fluid or clutch PAKS are slipping hence will need a rebuild.
sorry.
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