Volvo Repair: 1991 740 intermittent starting problem, neutral safety switch, jumper cables

Question
I have a 1991 740 non-turbo with 111,000 miles. For the last year I have been having this problem. A couple of times a week when I go to start it, the car does nothing.  The engine does not crank and the starter does not click, but everything else works.  The lights, horn, radio, warning lights and all electronics work.  If I let it sit for about 20-30 mins it starts right up.  There is no regularity to when it happens. It does this after sitting all night, or after driving all day. It does not matter if it is cold or hot out.  I replaced the battery, battery cables(pos and neg), checked and cleaned the grounds (including motor grounding straps), replaced the starter and solenoid twice and replaced the ignition switch (in the steering column). I considered the neutral safety switch, but when this happens I run the gear shifter through the range of P,R,N,D,3,2,1 a few times and try again. I have also tried starting in neutral. None of this helped. When this occurs I have checked with a volt meter and have power to starter(approx 12 volts), but still get no crank.  When this does happen and I cant wait 20-30 mins, if I put jumper cables on and jump it, the car starts right up. Please help as I don't know what else to do.  

Answer
is the 12 volts to the small sirew at the starter when cranking the car over this is the supply from the ignition through the nuetral safety switch.it may be a bad starter still.I would check for 12 volts at the small wire to the starter if this is ok then it is more than likely a bad starter.