I don't know that I solved any problems yet, but the kia seemed noticeably peppier this evening. In fact the radio even sounded better to me. I revisited G100, 101, 102, 104, 200, and 201 with some sand paper and dielectric grease. Honestly I never before thought to check out what was happening under the battery tray or the three individual grounds at G103. The plate beneath the battery was all heavily painted. It seemed like a bad way to ground the battery. I mean this thing has been running for 23 years, so it can't be that right? Anyway I took the dremel sander to the body and the plate that connects to the battery ground (G105?) and then made sure to disconnect each of the three G103 wires and sand that plate and each connector a bit before reattaching. I have previously done this for G200, 201, 400 and 500 as well. I didn't realize there was a G401 (defroster), so I did that too. I also used some connector cleaner for just about every accessible connector near the engine including three fuel injectors. Then I did some inside as well - mostly under the dash.

After putting the battery back in the car, I drove around a bit and something felt strange. After a few passes around the block, I parked and the CEL came on. I turned the car off. Then restarted. 30 seconds, then CEL. Then I remembered removing one of the plugs I put on a random port on the intake manifold. After replacing this, the car ran well. As I said before, it felt noticeably more responsive. Of course, since it is often months before I get this intermittent issue as described earlier it will be hard to know if I helped the situation.

Here's a few questions -

Would voltage drop due to poor grounds and/or bad connectors cause something like the fuel pump to momentarily stop working and then start again? Or cause, I don't know, say the ignition coil to quit for a second?

On a few occasions when the car has lurched, the engine kept running but it was sputtering in a weird kind of way - it felt like a bizarre electrical thing with the ignition. Could a poorly powered fuel pump cause this?

Finally, what's with the random port on the intake manifold? What's that for? It's very close to G104. Seems odd that it's there.

Chris, I picked up some relays from the PickNPull yesterday. I haven't tested them yet, but I will. The main and fuel pump relays in the car are the yellow ones I've seen online. They say "kia" so I'm assuming these are OEM. But just in case, I have these others. I suppose if I determine that the salvaged ones are ok, it will be worth a try to switch one out at some point if it's still acting up - just to see if that makes a difference.