Thank you for the replies. I'll try to address a few things brought up in order.

First, this is EEHack 4.93, which IIRC is the latest version. I helped with the whole "checksum mismatch" bug in earlier versions since this very car was the one that made that particular issue clear. So yes, this version of EEHack should be the one that's resilient against bad checksum packets (and in fact the error counter was over 700 before I stopped logging). So no, I don't think any of the data is bad, nor do I blame EEHack in any way shape or form for how the engine is running. I also don't think any of the data was a result of a noisy bus. The only exception to that are the random dropouts, which I cannot be sure are a result of bad data (since I know for a fact that jostling the cable in the USB port on my laptop can cause a device disconnect, and I don't recall what happens to logged data when this occurs). I should note, however, that despite EEHack reporting several supposed DTCs during these dropout periods, no DTCs are actually stored, nor does the SES light come on. These are entirely phantoms, seen only by EEHack, which further leads me to believe that it's not real but is rather either bad data somehow making it past the checksum, or a cable disconnect causing a bug in EEHack's display.

I agree that checking the low res and high res signals with a scope is an excellent diagnostic. But as you correctly assume, I don't actually have a scope handy. I do have a logic analyzer, but I don't have a way to crank it up to the data rate I'd need to make it emulate a scope on a signal like this. I could give it a shot, but I'll probably end up trying to acquire an actual scope instead. I've been meaning to get one anyway, may be time to pony up some cash.

The tach filter circuit is present and accounted for. I actually have the full maintenance records for this car stretching all the way back to the day it was purchased from a local dealership, and it's been kept in excellent condition. That's why I found it so strange that it was the one to end up with a random unexplained surge and a noisy data bus despite being in far better overall condition than my '95 (for which I have a grand total of zero records, and which has been beaten on).

kur4o, thank you for your excellent analysis as always. I'll go ahead and make the change to the BIN on the '94 to see if it trips the high res fail. I hope I don't have to do the opti soon on the '94, that would pretty much suck. My '95 required the whole kit 'n' kaboodle last year and that's been one hell of a bit of surgery. To then turn around and have to do the same on the '94 would be pretty much awful. As for swapping to coil, I do have a single DIY-LTCC here, but I was intending it for the '95 to test with. The '94 is my dad's car so I can't really use it for prototype stuff, which means I'd have to switch to an out of box solution, and currently there isn't one. Even if I wanted to pony up the wad of cash for Torqhead, they discontinued the C4 package last year.

Anyway, thank you all for the replies. It looks like I'll need to get myself a scope and start digging through some wiring if I want to get to the bottom of this mystery without throwing more random parts at it.