Last time I changed the coolant it took a few days to purge the air completely. I had to top it up quite a bit and suspected a leak, but it stabilised after a while.
Have you splashed any water over the coil packs or over the heads, where it might have gone down the plug holes? - or in the loom connectors at the front of the engine?
It could also be an intermittent fault on the fuel pump relay.
