You mean you turn the key and the starter doesn't spin?
Very common in lots of cars, Could be the Starter solinoid, starter relay, nuetral saftey switch, ignition switch, brake switch, bad ground on starter - Just to name a few off the top 'o my head ...
Your best bet is to get a mitchell manual or someone with alldata to print you out a flowchart on diagonistic for a 'non starter turn'
but since it's an intermittent problem all the more fun for you....