3.6 tdv8. Setting the cruise control at 120 km/h, on the slightest incline the car shifts down from 6th to 5th.

Putting it in manual mode and selecting 6th, it will comfortably go up and down any slope without a problem around 2k rpm.

I'm running 285/45/22s, is this messing with the gearbox ratios or can it be something else? Gearbox oil was changed last year by the previous owner apparently.

Although mine is a TD6 it does the same - on cruise it will often downchange & then back up again but without cruise it will stay in top, on the same bit of road at the same speed.

I found another thread on exactly the same issue and it seems that on cruise control this dropping of a gear is common. It seems that the car tries to keep the speed more constant by dropping to 5th even on the slightest incline, even though its so annoying.

I guess I'll just keep it in manual to stop that happening.