Ya'll will like this!
I have a 89 K1500 just installed a crate engine, rebuilt transmission and rebuilt rear end, ran great for less than a year.(less than a thousand miles) Went to the country to get some deer stands and my Black lighting started bucking and it wasn't not as strong, losing power. Made it back home and that where the fun began. Started throwing parts at her, than read on here the guy said stop throwing parts at it and start testing all sensor again, everything was reading correct. Spark plugs ok, Map 20 1.1 ok, pick up coil.939. Fuel pressure 12 psi new IAC and others, new cooling sensor, new ECM. I put a new computer in it when I put the new engine in it. I thought I was going to have to take it to a shop. My wife said take it to a shop, like most men on here this is your pride. Thought it might the TCC. I said I am going to rebuild the TBI then we will go from there. Went to take the bolts from the TBI intake, they were so loose. Right hand bolt was so loose you could see the dust ,carbon going straight into the intake from the bolt hole.
Taken out today and running great my question is:
1.Vacuumed leak under the TBI would cause sluggish and little jerking when giving it the gas? If so please explain, idles fine.
2. A crate engine, when should I do another valve adjustment? Thanks good men.

It was losing power and so the parts cannon comes out. Are you now wanting to test or throw more parts at it?
You say everything was reading correct. So no problem? Have you checked back pressure?

So just verifying: You tightened the bolts?

Vacuum leaks can cause all kinds of stuff, especially depending on where it is leaking. It leans out the fuel mixture being burned, and does it outside of the purview of the PCM so it can't possibly know to adapt directly. They also skew vacuum readings that are being used to make adjustments, so the PCM sees the results of more throttle opening, but random, and as you get closer to the edges of the maps, weird stuff will happen.

Assuming it is a crate 350, 305, or 262, and that is is a standard stock engine, you shouldn't need to adjust the valves unless the manufacturer says you need to.