I had a problem very similar with my truck. I changed out the ignition switch and it still did it. What it ended up being was a crappy ground from the body to the engine. I metered the voltage going to the coil and it was 12 vdc until I went to crank the motor, it dropped to less than 9 vdc when the starter was engaged and robbing major amounts of current. I ran a new ground cable from the bolt on the body down to one of the mount bolts for the starter and then just for fun another smaller one from the body of the coil/ic igniter unit to the body of the truck. It worked out great. The voltage drops a little when the starter goes but only a volt or two.