I will do some measurements next time I am racing. I did some reading and it appears that the resistance of metals increases with temperature. I suspect the resistance to earth is getting higher as he temperature increases. The negative terminal on the battery is connected directly to the engine and it does increase in temperature quite a lot (the engine). If this does turn out to be the problem I don't really know how to solve it, other than have a separate power supply, although I still need a common ground to read the analogue voltages.
The LEDs only stay on once it all gets warm, I don't have any caps on the data line to the strips, maybe I will add some.