Hi all -- problem solved, no help needed here but I wanted to write this up for future searches since I couldn't find much online about the P300 code. Short answer: one of the new plugs I had put in didn't work, which had the car running very rough.

I had the original plugs changed at 55k, and the third or fourth time I drove it after that it was misfiring like mad. The CEL give a code of P300, and the message display said "reduce speed and try to shift up" (even though I was going 30mph and the tach displayed 2000 rpm.)

The P300 code is for random misfire; otherwise, in a P30_ error code, the last digit corresponds to the cylinder misfiring (ex. P303 means the third cylinder is misfiring.) My indy mechanic (not the same shop that changed the plugs in the first place) did his assessment by hand -- switching plugs around to see if the misfiring changed cylinders. It did. I don't know why the code didn't show which cylinder misfired, but the problem is solved. Other causes for the code include a bad coil.