1997 Chevy Blazer Starts when it wants

1997 Blazer 6 cyl 4.3L The problem is: When I go to start the motor sometimes it works perfectly starts right away-it is all tuned up: new plugs, wires, filter, rebuilt alternator, etc. BUT sometimes I will turn the key and nothing at all. It will not crank-no life at all. I have changed the starter 2 twice in the last 4 years since I bought it. I should have picked up then something else was wrong the second time I changed the starter. I listened to the mechanics and they always wanted to replace the starter. It also has a new battery and battery cables. I put it on a scanner and everything was good: battery, alternator, motor, EXCEPT they said the amperage draw when starting the motor from the starter was way to high. The starter is new (within the year) and I had this same problem with the original and the last one before this one. Could it be bad grounds? The cables from the battery are new and tight. Maybe a ground cable from the motor to the frame or something else? It runs perfectly except for this problem. Also the AC sometimes works and sometimes does not work-I don't know if this is related and caused from the same problem? I checked for a parasitic drain but the draw when everything was off was very low so I don't think this is the problem.