Hello gentlemen, over the last couple months the starter on my 56 Tbird doesn't always engage when I turn the key. Most times it turns over, but sometimes I just get a kind of single click, then nothing.  It used to happen just once in a while, but now it occurs more often sometimes twice in a row. So of course I move the key back to OFF, and then try again. (I did convert over to pertronix electronic ignition a while back, and otherwise car has been running great.) I don't want this to become a problem....what should I check first?