Check Engine Light (but not all the time)

[B 4 You]

well, after a few weeks of what i thought was a random check engine light here and there, i figured it out. when i start my car cold, there is no check engine light, ever. but if i park with the engine warm turn it off, go get something to eat, and come back out, the light will be on when i start her when she's warm. but if i let my car sit long enough to get cold, the light will NEVER go on. not even if i drive really hard. is this a bad sensor? or is it serious? i am taking her to the SubeShop soon, as i need an exhaust leak fixed, and will let them see if they can find the cause. anyone else have (had) this happen to them?
(about a month ago, the light would come on at random times while driving, but would shut off after about 30 seconds. it doesn't do this anymore, but could they be related? also, the ABS light has come on 2 times. but goes off once i turn the car off and never comes on again for a few weeks.)
ugh. machinery. >_<