Why does the malfunction indicator/ check engine light come on and go off in my '04 jeep liberty?

Brian, your problem lies with the simplest of things, the gas cap. I'd bet that if you were have the code read, it would be a "Evaporative Emissions Control system leak" This is the gas cap allowing too much air to enter the gas tank. The code sounds really bad, but if you tighten the gas cap the next time the light comes on, It will go off a short time later. I've had the same problem on my Dakota, and I eventually had to replace the gas cap. $10 at the dealership, and no more light.

sounds like more than a fuel problem, better get it looked at. that also moniters the spark plugs, the transmission, all electrical parts and emissions, auto zone will do it for free, get the code and see what it is.

yes access the memory in the on board computer and find out why its coming on

In my Liberty this light came on because of a cracked hose on the charcoal canister. This light is normally part of the emissions system on the vehicle. One thing you might look into is a new gas cap. Maybe the seal on your cap is worn or something.