Seems to be getting worse: it bucks like a wild horse now nearly every time I reach 4,000 RPM. As long as I keep my foot on the gas at that RPM, it bucks. Otherwise, it still runs very well at all times...except for a barely perceptible roughness only at idle, which has been the case in many cars I've owned. In order to rule out the possibility of the Seafoam cleaning effect and also bad gas, I'm going to wait until the gas tank is below a quarter, and then go to a different gas station and fill it up.

It sure feels like the engine is either starving for fuel at that RPM--maybe an injector or the fuel pump (as I originally thought) or something electrical. I don't think one bad plug and/or wire would cause it to buck this badly at high RPM. But I could be wrong.