It sounds stupid but have you double checked the cap and rotor?
Mine had a mystery stumble happening. I tried alot of stuff valve adjustment checked compression it was all good.
It turns out when I put the new cap it wasn't aligned all the way and the rotor gouged all the points on the cap it also gouged part of the rotor off.
Pretty common issue the GP Soreson caps don't fit exactly like OEM so they have that issue.
I changed out te cap and rotor and made sure it was all straight when I put the new one on and now the stumble is gone.

86 Toyota 4 runner
35x12.5 BFG MT's, 4.56's XRC8 winch
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