I own a XC90 D5 2005 model

While driving on the motorway at 110km/h the transmission decides to change down to 4th gear, indicating a (-) on display with transmission service required.

When i switch off the vehicle, the transmission resets and i have a full working transmission.

But if i set the cruise control on a 110km/h and slow down to 80 and reactivate the cruise control, the transmission gears down correctly and does not get stuck.

I had the transmission oil flushed, without any improvement.

The error codes given by Volvo indicates slipping and that the SLU and S4 solenoids keep getting stuck, but why would it only get stuck while driving normally and not on cruise control

Any advise?