my car will start cold but after quick trip wont restart-wont crank

My car used to do this and it turned out to be the main relay that needed to be replaced. In the summer the heat would cause the relay to stop working and I would get stuck until the car cooled down.

When you say 'it won't crank' does it click at all? if it does 'click' do the dash lights dim , or go completely dark until you let go of the key?