Large RPM drop during runup

I’ve gone over the wiring a few times (including giving a gentle pull while measuring with the ohmmeter) and haven’t been able to find anything. We measured 2 of the trigger coil gaps and they were in the .012-.016” range; the other 2 don’t seem to be accessible without taking the engine off the plane :( 

The thing that gets me is how the behavior tends to get worse after just a few minutes of running at idle. Maybe a carb overhaul is the next step?