Master switch and starter relay are good candidates. But you need a few things to make the diagnosis easier: A multimeter: recommended model, and treat yourself to an additional set of leads with croc clips. Confidence that the battery is good. Measure the voltage across the battery terminals at rest, as you try to crank and at 3,000 rpm. The wiring diagram. If you haven't got one send me a Private Mail.In the meantime can you connect a good battery or another vehicle (best running at 3,000 rpm) to the battery with thick jump leads and see if that makes a difference. Jonathan