Any clues as to why my check engine light on my Sonoma would go on after filling my truck with gas?

Then after several fills later it has gone off, however crossing over some bumpy railroad tracks just the other day, it came back on and still is on. (Everytime I go to take it to the local Autozone for them to hook it onto their error code tester---the light goes back out --just before arriving there, so they can 't locate the source of the problem). The wiring has been checked and is working right, also the idiot bulb has been replaced. The truck has recently had an oil change, flush, and tune-up. I just rolled it over the 100 thou. mark......