Another thing to ensure that you never have earthing problems and have full battery power to the engine is to connect your earth cable directly to the engine (you will still need the earth strap between the engine and the body for the other electrical goodies). I have connected mine to one of the top bell housing bolts which means that the cable is nice and short and you can easily check your connections. Another thing you can do, as part of the fault finding, is to measure if there is any voltage drop across the earth straps. To do this connect a voltmeter between the negative post of the battery and the engine block. If there is any significant electrical impedance the earth circuit between the battery and the engine you will measure some voltage - a few millivolts is nothing.