Xride

I got a SES light the other night when I spun the tires in a parking lot, it went out for the next morning, but now when I drive hard, like get the revs up over 4k it comes back on, but the light will go out if I let the car sit over night.
I have tried to fidn a place around here that will scan for free, but have only found one place and its on the otehr side of the city, so it may be a few days befor i can get it scaned, also the light isnt on all the time, can they still find what code it was if the light is off?
What do you think it is?
I havent noticed any difference in driving at all.

95z28 -quicksilver

It could be a couple of things. How old are your plugs and wires? Also check the vacuum hoses on the opti. Or you could have a leak on your intake manifold.