Engine stumbling at 4000rpm

[doublew]

My Toyota 1.6L 4AGE with mods (Haltech E6X, HKS ITBs) has been running well. Until now.

Stationary, with the trans in neutral, the engine stops climbing around 4K RPM at partial throttle. I see AFR reading 13 until then, then coinciding with the sputtering it jumps to 10AFR then back up then down and up.

The car struggles to go up hills lower than 4K as well.

Either of these cases, I can usually can get it to work better by pushing the throttle in all the way. But it just stumbles a bit less.

I checked compression, all of them around 179PSI with cyl 4 being a bit less than the others

1 181psi
2 190psi
3 179psi
4 169psi

Checked spark plugs, they seemed a bit black, used a butane burner to burn off all the soot. I have a pack of new spark plugs that I can throw at the problem if needed.

I checked wiring, there was one wire that was loose from a bad crimp, fixed that. Did some random things around the engine too. Drove the car around that day seemed like it fixed the issue. But a day after the 4K rpm stumbling came back.

Checked vacuum and it seems like one of the vacuum lines was barely sealing at the junction nipple, fixed that.

I see inj duty cycle jump up and down during the 4K stumble.
I'm using MAP as load and the signal seems constant.

How can I debug why the inj duty goes up and down?