Most likely while you are driving one of the height switches opens as if it was in a lifted position causing the code. The code appears because one of the height switches opened and the switch below it stayed closed.
Switches do wear out causing them to work intermittently or only at a certain pressure on the switch. I see it quite often. When you test the switch, it tests just fine. Once you start to wiggle it or even slightly let it go the resistance jumps way up causing it open early.
It beeps because the lift thinks you are in the air when the switch opens causing the lift to set off the gate alarm. Even with a missmatch the lift will assume you are in the air and will require saftey features.