Gretings, Former Service Technician here,
I used to repair these instruments. the OVERSPEED error is a quick way for the instrument to tell you that the bar code on one of the internal glass circles cant be read properly. There are three causes for this:
Instrument turning too quickly
Instrument cracked, and light getting inside of the area that is required to be dark in order for the encoder to be read properly
Dirty encoder glass
Id recommend finding a local Trimble/Nikon Service Center and taking the unit in for service. It's easy to resolve with the right equipment.
I hope this helps. If you are in the North East area of the United States, Keystone Precision Solutions has you covered. www.keypre.com
Good luck!
-Robin Butler
Construction Field Sales