You most likely have a speed sensor problem. An OVP problem is unaffected by recycling the key. Any intermittant capable of setting the light will also be reset with an ignition cycle, just as any such fault will always keep the light out till recycled.
Since the OVP powers the controller when it fails the controller sees it as a recycle. So the light can come off and on and not be viewed as a fault to the controller as it just thinks it is being turned off and on.
When the light is on the system is off and won't function.
Bosch Master, ASE Master, L1
33 years MB technician