I've been having some real problems with my 99 tahoe 4wd 5.7. It started about 2 days ago and I'm baffled. My truck began being iffy about immediately starting which I thought was odd but gave no major concern until it started cranking and cranking but not starting up. The batteries seemed strong but I thought what the hell, lets try a jump. It immediately fired up and I went about my way. Fast forward a few hours, the same thing occurs. I then thought it could be a clogged fuel filter so I swapped that out, primed the line by turning the ignition several times before starting, still a no go. although it would sputter trying to start but not fully fire up. After waiting about 10 min I gave it another shot and it fired right up. The next morning I tried to start and it cranked and cranked and finally started, but going down the road it died once again. I pulled over into a parking lot, waited, and fired it up again. This time there was no hesitation but the moment I took it out of park it died again. I played with the batteries disconnecting the second battery leaving only the core battery and it fired up again. I immediately started making my way to autozone where it died right outside in their lot. I took both batteries in to charge and test and both came back ad bad, so I bought a new one, threw it in, and it fired immediately twice. I was just about to be on my way but decided to plug my small inverter into the cig lighter, and it immediately died. This was accompanied by a slew of new problems. The truck now won't start, and anytime I crank it, everything in the cabin resets as if the battery was disconnected. Now with an 80% charged battery, the windows are going up and down sluggishly and may even stop entirely. If I turn the key without cranking, all the bells and whistles reset and the "light warning alarm" that comes on when starting is weak and reduced to a whimper, the dash clock and gear indicator light are dim, but, if I turn the key back to the off position and back on, everything comes on with full power but the truck still will not start. I'm at a total loss as to what is causing all of this and don't have money for a shop. The autozone guys tested the new battery while installed and it's delivering all the cold crank amps it should with no results. Please help!

 

Could be a bad ground. Clean and check all ground wires you can find.

 

You have a very poor connection somewhere.
Need to track the battery cable back to the starter and check for corrosion as well as where it connects to the main fuse block, the wire needs to be clean and not swollen with corrosion.
Also inspect the ground cable from battery to the engine and chassis, the wire needs to have no loose strands and no or minimal corrosion, check for any ground straps running from the body to the engine.
Check a few remote locations like the dome light or rear cargo area lights and see what voltage you read at those points, it should the same or nearly the same as at the battery (checking terminals with lamp removed and the lamp switch turned on)
if you see a large drop in the voltage you need to find where the poor connection is.
You can also ohm out the ground connection with a meter, check the battery neg connection to different points on the engine and vehicle body where clean clear bare metal is available or a ground connection can be checked.
The resistance reading needs low, like 0 to 7 ohms, lower is better...

 

Just a thought, starter or ignition switch?