Bmw523i. Y does auto trans stick in 4th gear even when takin foot off acc 80mph 5000rpm acc slow goes thru gea

When driving normally it will change gears at between 2500 & 3000 rpm.When in 6th gear,and overtaking,accelerating fast.it will drop back to 4th,and goes up to between 70 & 80 mph,taking foot off accelerator to drop back to 5th & 6th gear,it sticks in 4th and will not go back to 5th,6th gear until you go back down to 60mph,all the while not touching the accelerator. This is happening 2-3 times a day on an 80 mile journey,previously it has only happened 3 times over 14000 miles of journeys.current mileage 114500, 1998 model

Answers:
The first thing to check is the transmission fluid level. I believe on your car, you have to remove the bolt from the top up hole and add transmission fluid until it starts to overflow.
The reason I think it's this is because accellerating will wincrease the oil pressure in the 'box enough to change up but when you take your foot off, there is not enough reserve fluid to activate the change down.
Whatever the case, this is always check 1 in the mechanic's handbook.
sounds like valve block in the gearbox is sticking has the box ever been serviced?usually one of the things that tends to be neglected on a otherwise looked after car. also get a diagnostic done on the box could be a sensor gone down as it is controlled by its own ecu

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