I dont mean to insult any ones intelligence but if timing hasnt been moved have the plug wires been moved? Maybe re-check the plug wire order?

If I recall correctly, when I disconnected my cold start injector to test something and forgot to plug it back in, my motor wouldn't fire up cold. Maybe make sure the cold start injector is plugged in and working. Another possiblity would be the cold start injector working too well and flooding the engine.

Another possiblity is a really weak coil not producing enough spark.

Of course any major leak in the intake system like the intake hose not fully installed and sealed will cause a problem, but so will if you have the oil cap or PCV valve remove or any vacuum lines disconnected.

These are just some things I would check if the other suggestions don't pan out. My shots in the dark. Good luck.

I may as well throw my 2 cents out there from what I have seen. IF timing is off you can get backfire through intake, (was mentioned). If intake valve is incorrectly adjusted to too tight, you might get backfire through intake. If you (and I have had this)have water in gas you will get bucking engine, backfire in intake and an engine that hardly runs and sounds like its about to come apart. You describe my truck after I sucked in a ton of water that was in teh fuel tank, I really thought I had a mechanical engine problem, sounded like pistons or valves hitting either each other or something elese inside.