The cars computer monitors well over 100 diagnostic points for your engine, transmission and other equipment. The CEL trouble code(s) should be stored in the computer for an amount of time, and is retrievable even though the light may not be on at the particular time.
After you get the CEL light, take it to Autozone where they will give you the particular code number(s) that is displayed or stored, as well as a brief description of the problem (i.e., "insufficient EGR flow"). You can also check the internet for further info. I may be able to supply you with the list of codes also.
It sounds like an electronic problem to me. I had some trouble similar to this once when I degreased the engine. I got water all over the connectors and devices on the outside of the tranny and it acted funny until the water dried. I'm not saying this is your problem, of course, but it illustrates the effects of electrical type issues.
A fluid change should not hurt anything. It's wise to do the drain/fill/drain/refill procedure about every 15 to 20K miles, or less.
I use Mobil 1 ATF in mine with some Lubeguard additive, which is very good from what I've heard.
Mechanical problems, however, are a whole different ballgame.
