mystery engine light

I have a 2002 gmc savana, every so often the engine light will flicker on and the engine will hesitate, but there will be no codes stored for me to pull out with my code reader, there is no specific time when this happens, hard acceleration, down hill, or cruising. any suggestions out there for me to start looking at.
thanks
robert

actually the engine light was an old stored code that went away and never returned when cleared. I changed the trans filter and oil and have not had a problem since. just thought you guys should know.