Occasional flashing 'Check Engine' light on 2002 Prius. Possible causes?

I have had a 2002 Prius since December of 2012, and it has been running fine since I got it; while it was in a wreck in May 2013 that has rendered both passenger-side doors and windows unusable, I have not seen that incident cause any other problems, and the car still runs perfectly fine. I am supposed to be driving from my current home state of Colorado to New Jersey for a job soon, but about a week ago while I was driving on the interstate, the check engine light started flashing; I was changing lanes and putting a good bit of pressure on the gas in order to pass someone that was going below the speed limit, putting my speed at around 80 mph when the check engine light started flashing. I started getting off of the interstate to find a place to stop and inspect things, but the light stopped flashing when I got down to around 45 mph on the exit ramp, and I was able to drive the rest of the way on the interstate without any trouble, even when I got in the 75 mph range again. The span of time between the light starting to flash and turning off again was probably about a minute or so.

The check engine light had not turned on since then until last night, when I and a friend were driving to another friend's house; we were going around 66 or 67 mph on the highway when the light started flashing again, but it turned off after about 5 or 10 seconds once I slowed down to around 60 mph; for the rest of that first drive and for the drive back, the check engine light did not turn on again, even when I got back up to the same speed that had caused it to come on.

The check engine light has occasionally turned on for me before (although it stayed solid instead of flashing), but that was simply due to me not letting the car warm up for long enough before driving it, especially in cold weather, and it always turned off again within a couple of days of turning on if I warmed the car up. I have not heard any strange noises or felt any differences in driving my Prius whenever the light was flashing on both occasions that I mentioned, but I am getting nervous about the issue since I am going to have to drive across the country soon, and I don't want the car to suddenly die on me in the middle of nowhere. Does anyone have any guesses or ideas as to what might be causing my check engine light to flash like this, and how urgent of a situation it might be?

 

My odometer reads 113,939 miles. I doubt that it would be a loose fuel cap, as I always tighten the fuel cap until it clicks several times. And I'm not sure when the spark plugs were changed last; I have not had them changed since I got the car, though.

I did call my nearest Toyota dealer a few days ago to ask them if they had any openings to look at my car, but they are booked solid for two weeks, and I am supposed to be making the drive sometime this coming week.