Not your typical valve adjustment question
 

I'm hoping someone here can help, I'm out of ideas and at the end of my rope. Sorry for the long post, but I want to provide as much information as possible to try and get an answer.

First, the particulars: 383 SBC with Edelbrock Performer RPM 6089 cylinder heads and Scorpion narrow body self-aligning rockers (They can be viewed here). Because the rockers are self-aligning, no guide plates were used.

There are a million posts & articles about how to properly adjust the valves. The method I used each time was to bring the #1 cylinder to TDC, then adjust 8 of the valves (I don't remember which 8 right now, but they are listed in many of these articles). Then, I hand spun the engine 360 degrees to bring #6 to TDC and then adjusted the remaining 8 valves. I'm 100% confident that I'm adjusting them correctly, save for what I say about my 0 lash below.

When I was first trying to get the car running (in October), one of the poly locks fell off and a rocker came loose. At that point, I assumed the engine builder I used didn't have it on properly, so I went and readjusted all 16 valves per above, at zero lash I tightened the nut another 1/2 turn, tightened the allen head to lock the nut down and done.

After barely driving the car all winter, I recently got it on the road. About 3 weeks ago, I heard one of the lifters start ticking. About a day later, the tick became a loud banging - another one of the poly lock nuts came off while driving, as did the rocker arm.

I took both sides apart, followed the same procedure as above and then this time turned the nut an extra 3/4 of a turn. Got done, car ran like crap & I still had some noise. I checked cylinder temperature, one of the cylinders was much colder than all of the rest. So I took the car to my brothers' shop and started over - following the same procedure as before, I adjusted each valve, had him check them (he's been a mechanic for 30+ years), then turned the nut 1 full turn extra. Got done, started it up - and they are still making noise.

Now, here is the odd thing - when adjusting them to 0 lash, there is definitely no up-and-down movement, but for lack of a better term the rockers rock back and forth. So basically, after you adjust & tighten it down, you can still grab the rocker and move it pretty easily from side to side, like if you were trying to twist it. My brother said he's never seen that before, but he doesn't have a ton of experience with full roller rockers.

When completed today, the car ran a little bit better but the noise was still present. It honestly doesn't sound like a typical lifter tick but more like a rattle. I have no idea if (1) the builder installed something incorrect as I've found multiple issues with other work he did, (2) there is something I did wrong, i.e. my 0 lash should be where the rocker doesn't move at all or (3) the heads and/or rockers aren't right for my application.

If anyone has any thoughts on any of this, please let me know. Thank you

The text book way you've been setting the lash is fine, as long as you are following up with a final adjustment "the engine running way". As a "set and run", that procedure sucks and I'm not surprised that you're ending up with a loose lash. You want to adjust the valves at the heel of the cam lobe. Here's one way, with a good description as to what's going on: Engine Valve Adjustment Procedure - The Right Way
Where is the movement with the rockers coming from? The valve stem fits between the tabs on the rocker arms. Is the slop between those points, or within the rocker arm itself?