Car engine temperature controller resets its memory

[HonkMaster]

Hi! I have a weird problem. I made an adjustable circuit that turn on radiator fan on when engine reaches a certain temp(90degree in my case) and stop it when engine temp reaches a lower value(80 degree for this car). This is for a modified off road car.

I used a simple electronic thermostat that have a user interface for adjusting temps and other settings. The onboard relay is only 5A rated, that's why i used a 12v 40A car relay after it.

The  circuit is working fine for 2-3 h, but suddenly it's loosing all programed settings. This is only mounted in car,
I test benched it, without the 12v relay and other addons. Just 12v noninterrupted for 2 days. I used a mosfet and a Arduino board  to turn it on and off every 1 min for 20h and no memory loss. 
I resoldered every connection, and added more caps for filtering, but the problem is still there.

I was thinking that some interference form the car electronics are causing this. First culprit is the fan relay that is located right behind the module, but I'm not sure. Any advice will be helpful. Thanks!

[floobydust]

Many things can cause the problem you are having. From the partial schematic, there is no transient protection and not such a great circuit so I'd expect it to not work properly.
Where is the temperature sensor, is it one or two-wire? Your grounding can also be an issue.
Is "onboard 5V reg" a 7805 or 1117?
C4 is not done right, it is supplying for the entire car and not just your board, as well as hard on the ignition switch arcing to charge it.
Do you have a back-EMF diode across RL1 coil? or RL2 coil?