Might be relevant, might not, I started a thread about it a while back. Garmin edge 1000, logs rides fine, mapping great but would it follow a route would it bollocks, just kept crashing or giving me wrong turn instructions. Tried factory reset etc with no joy. Two telephone conversations with Garmin they asked me to hook it up to a laptop and they would send me a link to send a zip file to so they could assess the problem. I don’t own a laptop only an iPad so that was a no go, they offered me a replacement for 200 quid which I declined.
So here’s how I sorted it ( through luck really ). I was using an OS 1:25000 map which I purchased from eBay but I also had a Talky Toaster SIM card which I replaced the OS one with. It now works perfect, turn by turn navigation, never missed a beat since.
When I have some spare time I’m going to put the OS card back in just to prove it was this. HTH.