GPS tech (Garmin, Hammerhead, Wahoo, RWGPS, etc)

Any suggestions as to how to fix it?

Factory hard reset? If that doesn't fix it, then I doubt anything else will.

Are garmin likely to offer an exchange or replacement at some kind of cost?

Dunno, but if they do offer anything I suspect it will be a different more modern unit at a modest discount.

If not, what is the forum approved cycle gps/computer these days?

Here's my experiences on the ones I've used recently (ignoring the Garmin 500/800/810 which aren't available and the 520/820 which are just shite):

Garmin 1030 - overall excellent, stable, good screen, capacitive touchscreen is still a PITA though. Rather large and stupidly expensive. Good for nav, although IME the routing using the device is hopeless.
Garmin 130 - small, cheap, stable. No good for nav as the screen's too small, and the buttons are flimsy and tend to get stuck down.
Wahoo Elemnt - stable, works well, routing is fine as long as you have your phone on you. Good large screen, but the altitude sensor goes all wobbly after a while and turns into a random number generator, I suspect due to the barometric pressure ports getting blocked.
Wahoo Elemnt Bolt - like the Elemnt except the screen's too small for navigating except on very simple routes, and the altitude sensor doesn't seem to go wobbly presumably because it's covered up by the aero mount.
Karoo Hammerhead - fantastic screen, versatile device. However, the firmware's still a bit flaky (albeit not as bad as the abomination it shipped with) and it's useless if the Karoo website back end goes down, which is something of a concern as I'm not convinced Karoo is going to be around for ever.

@Cycliste has an 830 and seems to like it - personally I'm tempted by the 530 as I've grown to loathe capacitive touchscreens on cycle computers given that they can't tell the difference between my fingers and beads of sweat or drops of rain.

tl;dr: Garmin 1030 if you want nav, Wahoo Bolt if you don't.