My Plymouth runs great right after I start her up, but after about 20 minutes of freeway or surface street driving (after heating up to normal operating temp), she starts running rough: The idle drops, and when I take off from a stop light under normal load, she backfires a few times through the exhaust.

I'm running a 413, MSD-6AL, MSD wires (new), Speed Demon 750, recurved FBO distro, new FBO coil, Accel header plugs (I've been running them about a year now - 500 miles), and 101 octane pump gas (real pump gas, not octane boost). Timing is set to 38* (per the distro spec sheet).

Any thoughts?