Crank sensor cuts out above 3500 RPM

So I've been doing a bit of coarse road tuning just to get my car up and moving, and that's been going decently, but as soon as I get above 3500 RPM, it starts cutting out, going back and forth between reading the actual RPM and reading 0 RPM, sounding like some scuffed version of 2-step. What could be the issue here?

And since I'm posting anyways, bonus question to which I assume the answer is ''yes''; Could a fluctuating AFR (going back and forth a few 10ths irregularly) be caused by the fact that my engine is on the tired side (leaky valves, worn bores) and the exhaust isn't fully air-tight anymore?

I'd expect it to be a wiring/connection issue, but it could be something like an air gap, or whatever.

What happens is you manually wiggle the wires when it's held at, say, 2k rpm?

With the AFR, any issues with the ignition and/or injectors, will cause the AFR to change - it would most likely be lean spikes, as either an injector failing to open properly, and/or sparkplug misfire, will allow un-used oxygen into the exhaust. If it's rich spikes, it may suggest a leaking injector that isn't closing properly.

The wiring is question is a Plug-and-Play harness bought off a reputable guy, so I doubt it's the wiring. I will test it to make sure, but I suspect that's not the issue.

About the air gap; what would a good or ideal air gap even be? I've wondered about the alignment and distance between the VR-sensor and the trigger wheel before, but since I have never seen any of that talked about, I figured it would work with a wide range of setups and never gave it any second thought. The harness I bought did include a bracket for the VR-sensor, but the spacing could be off by a few milimeters, could that potentially be the issue?

As for the injectors and coils and stuff, all that stuff is brand spanking new, so I doubt that would cause the fluctuation in AFR. Nothing is impossible, but it seems unlikely to me that new hardware would cause such fluctuation.

In these cases an oscilloscope often shows you exactly what's going on. You can record all triggering signals, see if one gets dirty, has drop outs etc.

Some ECUs have the function built in. Does yours?

Sensor air gap is typically 0.5mm to 1.0mm, trigger wheel has to run true without runout. If you're off by "a few millimeters", there is your problem!