For the past month or so my mustang has periodicly displayed a "Check engine light" it usualy goes when im on the highway and will stay lit for a day or two and go back off. At first i thought it was the gas cap, but taking it out and reclosing it doesnt seem to help. anyone have the same problem or ideas on what it is? i would take it to the shop but it never stays lit long enough. I dont think its serious but still makes me peranoid.