The automatic transmission shifts from 3rd gear to neutral while off-roading.

[MBHH]

At the weekend I was out and about in an off-road area. I was driving in 3rd gear (manual transmission) in off-road reduction. The Grenadier had to work hard to drive up a steep slope. Suddenly, the automatic gearbox jumped from 3rd gear to neutral in the middle of the drive. The car stopped, of course. I was able to engage reverse gear and drive back. This phenomenon was repeated several times that weekend. Have any of you experienced this?