I had the same problem until recently. All I did was remove the starter motor, which wasn't that difficult, dismantled the solenoid, and cleaned the contacts. Bongo starts first time every time now, but I know it's only a temporary cure as the contacts were well worn. Soon, when I have found out what model of starter motor I have, I will be contacting this company
http://www.nationsautoelectric.com/densoparts.html
to get some new contacts, and then I will replace the old ones.