Yeah, checking mileage is a good idea too. That'll give you an idea on how its running. You ought to double check as much stuff as you can. Does it bog just at low RPM? How's the high end, around 4000 RPM? Is it strong up there?
It's kind of weird that it ran so strong for a few days then just lost power. As others have suggested, disconnecting the battery for a longer period of time may help. Lots of other things it could be. Double check all those vacuum hoses, the wires to the O2 sensors, stuff like that. One of them may have melted against a header pipe or something like that.
I think if the tranny was going, you'd notice some slip, especially when it changed gears. That isn't always the case, but it is the most likely symptom.
You haven't gotten the new chip in yet, have you? Like others have said, your computer may have been fooled, in which case a chip will definately help. You may need a custom burn, though.
And finally, when you fill up next time, use premium fuel to see how that works. It may take a few drives to get any results, but its worth a try.
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