You might want to check your engine ground strap. I had a similar problem on my 99 Chevy Tahoe. In the Chevy design, the starter and ground wire is located too close to the exhaust manifold. After chasing the electrical gremlins for awhile, I removed the engine ground and found it to be brittle and corroded. Since most of my work is on boats, I used a larger size marine grade wire and some heat reflective tape to replace the engine ground and starter wires.