The next Morning, I went to start it and it would not start?

I've been having the same issues since a few days after I bought this car and it's been 2 years now. 90% of the time I've owned it, it has been sitting.

Basically, I bought it from a private owner who posted it online. Immediately after, I took it to a Chevrolet dealer and had them do a tune-up and change all my fluids and filters. I drove it to Tahoe (approximately 150 miles), and it ran like a champ there, all around there and back! I came home and it sat overnight but the next morning I went to start it and it wouldn't start. It wouldn't even crank, nothing!

I had to go to work but when I got home, I tried to start it again, crazy but, now it would crank but it wouldn't turn over. Turned the key on without trying to start it and couldn't hear the fuel pump. Checked the relay and it was good. So had it towed to a shop recommended by a friend of mine.

The mechanic said he put in a new fuel pump and that didn't work so he changed the relay and he said that he did it. Now I'm back on the road and everything was good, it was running fine for a few days.

A few days later I went to the store and when I came out, it wouldn't start, wouldn't crank, again nothing! I called a friend who came to the store and tried everything. Lastly, he pulled the lever up to the computer harness on the floorboard right inside the passenger door, waited a minute or so, pushed it back in and voila! It started right up.

Fast forward a few more days and it started but sputtered loudly and died out. Turned it off and restarted it, it ran just fine.

About a week later I ran to the store, came out and it wouldn't start back up. It would crank but wouldn't turn over. Tried a few times and the fourth time I tried; it wouldn't even crank. Again, nothing. So, I lifted the lever on the computer harness like my friend did wait a minute, pushed it back down and voila!

The next morning, I tried leaving and it wouldn't start, it tried to, but it would shake and run really rough and then die out. Happened the remainder of the day and the next morning. A neighbor of mine (mechanic) said it was the mass air flow sensor. Got a second opinion, said the same thing without me even mentioning the first diagnosis. Switched that out and still did the same thing.

I was fed up, left the car alone for about a week. Had my friend come look at it, tried to show him what it was doing but it started right up like it never had a problem. Was the craziest stuff!

Not long later (days, a week or two maybe) doesn't want to start again. Can't hear fuel pump come on, relay is good, fuses are good. Get a few opinions and everybody says fuel pump. After a few days of sitting, try to start it and it started up, shook and ran rough just as if it were the mass air flow sensor and died out. Turned it back off and tried again and It started and drove just fine!

This has been the same cycle over and over. It's not the fuel pump, it's not a relay or fuse, it's not the mass air flow sensor, it's not the ECM. I don't know what it is. When something gets messed with and then it starts it makes you believe that you found the problem, but it always comes right back.

I forgot to mention, 4x out of 10 when it cranks but won't turn over, the key gets stuck in the start position and will continue to try to start for a second or two even when I let go of the key

Also, sometimes when I'm trying to start it, if I turn the key on and wait for the security lock on the dash to go off, then try to start it, if it doesn't start that light comes back on as if it's the security system preventing me from starting it.

I don't understand it at all. Sometimes it will start and sometimes it won't. When it doesn't sometimes this will work, sometimes that will work and then when nothing works just let it sit for a few or two or more, then one day I'll try it and it will fire right up.

This happens over and over and every time it does start, it runs great! Somebody please help me!