2000 frontera problem?

i recently brought a 2000 frontera b auto, petrol/lpg, which the speedo is working intermittently i have taken the speedo out and checked it and it has had a new motherboard in recent months, also when i switch on my light the bottom jets on the lights start to spray screen wash could this be a problem with the earth wire? if so where do i locate it please? my car also on odd occasions cuts out when i turn the ignition on, the rev counter shoots up then cuts out completely what could be the cause of this? ( yes its got fuel in both tanks ) any help would be greatly appreciated thanks