Starting Problems

[softedave]

I have a 2008 Roadsport that I built myself. I have never had any problems before but now gave an intermittent starting problem. When I click the start button sometimes the starter fails to turn and the immobiliser re-activates. It sounds and feels like the problem you get if a battery lead is a bit loose. However, all connections appear to be good. Also, when the problem occurred today I kept my finger on the button and it made a humming noise for a few seconds before starting to turn the starter again. Any ideas?