Welcome to Justanswer.com, my name is Dustin.I'm sorry to hear about the issue you are having, I will do my best to help you resolve it.
We need to reset the unit, and then update its firmware. heres how to do that.
First, the reset process https://www.youtube.com/watch?v=OWaEX4jWtog
And then, to update the firmware go to this link http://www8.garmin.com/support/collection.jsp?product=010-00717-20
Download and install the newest version of Garmin express, and then once installed. Run garmin express and install any and all updates it shows for the nuvi 255. Once complete, test the unit. Is it working any better?
I strive to provide great service, and greatly appreciate a 5 star rating clicked and submitted when you are satisfied.. I always answer follow up questions.Dustin S.Senior Technician and Network Switching Systems Operator / Maintainer.