Vauxhall Corsa all electrics

hi im having terrible problems with my corsa whenever it rains all the electrics go crazy the service warning light comes on if i indicate the head light adjustor kicks in, the headlights switch to full beam even thought the lights are actually switched off, it looses power then accelerates like crazy then dies again, now the abs light is on and wont go off and the speedo and fuel guage doesnt work now they have run it through various diagnostic tests and its says there is absolutly nothing wrong. A few months after getting the car a hose spilt and it over heated it blew the cylinder head gasket but no warning light came on to say it was over heating it seems like there was already problems with the dashboard even then