Not familiar with this specific rake but ground driven equipment in general can suffer from accumulated looseness, worn splines, missing shims/pins. Gravity is having some effect if you only have the problem when on a side hill. Some part is also sliding downhill. Do you have to re-engage manually or does it self correct? Does the drive shaft still turn when this occurs? Is there a missing detent ball or weak spring that allows the transmission to come out of gear? Is there a missing shim / spacer allowing gears to disengage? Are wheel bearings allowing the whole rake to slide slighty downhill away from the drive wheel? Here's a parts manual for short money that might help you see where the problem could be. New Idea 4150, 4151, 4160, 4161, 4170, 4171, 410, Hay Rake parts MikeD74T
