The ability to lock and unlock only some number of times is only a factor on cars using the engine for vacuum and the engine is not running. The 126 chassis uses a vacuum pump and should lock and unlock till the battery runs dead.
The pump is designed to shut off after a certain pressure or vacuum is achieved each cycle. If the system has a leak and the pressures aren't achievable then the pump shuts off after some time period. I suspect something along this line to be your problem. To test locate the pump in the spare tire and watch what happens. Does the pump shut-off after similar intervals both locking and unlocking? If the time is long try removing the line and plugging it and see the time length.
Bosch Master, ASE Master, L1
33 years MB technician