Sometimes it dos'nt get the GPS signal and at other times the screen has a dark blue grey tint at medium chart sizes.
If the screen has the tint only when certain chart sizes are displayed this would tend to rule out any problem with the screen itself. If the screen was at fault, the tint would be there all the time.
If your charts are on a removable card (SD card, C-Map card, etc) the card itself could be at fault. Or the display software in the unit might have developed a fault that causes the tint to be displayed when certain chart sizes or parameters are selected.
Temperatures can indeed play a role in how a piece of electronics acts. Don't know if you keep heat on your boat but if not, try warming up the cabin (if you have not already tried this) and see if this has an effect on the problem you're experiencing. Also try turning the unit on and leaving it on for several hours. It may be a matter of some condensation breaking a connection or causing a problem on a board inside the unit. Leaving the unit on might "cook" the condensation away.
An intermittent GPS signal could be a simple as a bad or slightly corroded connection at the antenna or display unit end. The power we're talking about here is very low, so it doesn't take much resistance to stop the flow of electricicals
We've even experienced this once when our transom zincs were replaced. One new zinc began to deteriorate as normal but the other one remained pristine. Continuity all checked out and it wasn't until Paul Graf (you know him, right?) mentioned the very low current involved and how even a tiny bit of corrosion or coating on the bolts could break the connection to the zinc that it occurred to me to remove the zincs and clean the mounting bolts thoroughly with bronze wool. I did this and the problem has never recurred since. So check your GPS and display unit cable connections if you have not already done so.
