Cadillac Repair: 1996 cadillac deville wont start, neutral safety switch, cadillac deville

Question
This is my third time posting this question.  I am not sure if they are going through or not or if you are just that backed up with questions.  If this works I  would appreciate any feedback.  I have a 1996 cadillac deville that won't start all the time.  The lights work and the stereo but when you turn the key all the way nothing happens. it does not try to turn over, just one click is all you here.  If you shut the door several times and try it will work or if you try turning the key enough time it will work.  The shop I Purchased it from put a new battery in, replaced the possitive terminal and connections to it and checked the alt. and every thing is charging fine but it still  wont start all the time.  It will start fine for three days then at random it happens.  Oh I was also told that the positive connection at the battery keeps coming loose so they told me to just keep it tight.  please help.  

Answer
Hi Barry, Hopefully this email will find you because I have never had a problem like this before and am not sure why the other 2 messages never reached you.
It would be so much easier to fix if the shop could test things when it is acting up but realize from a tech standpoint that might be tough to do. My first thought is a defective ignition switch...corroded or partially dirty contacts. Useing a voltmeter at the starter purple wire should have battery voltage and less than that with a no start might prove that theory. The neutral safety switch on the trans might be bad. Did you try starting it in neutral and what happened? Connections at eithor end of eithor battery cable could be dirty or corroded. Remove all the cables and clean then as well as the engine ground spot and starter bolt. Make sure the bolt on the solinoid is tight. Lastly maybe the starter has a bad spot on the armature, worn brushes or the solinoid is weak or doesn't want to work when hot. As for the pos. battery cables they recommend a washer be placed under the battery bolt so the bolt doesn't bottom out in the battery hole and that would allow the bolt to tighten more. Sounds like someone is going to have to catch this at the correct time because once the loss of voltage is located the problem can be narrowed down and fixed. Good luck. Bill