There is no filter change interval for the CD4E. The filter is located inside the case of the tranny and to change it you have to drop the tranny, which means dropping the subframe, and crack the case open.
As for changing fluid - if you look under the car on the drivers side behind the front tire, you will see a drain plug. A 3/8 drive rachet will fit right in there. You pull the drain plug and allow it to drain. You will only be able to drain 4 quarts, so you will have to drain, reinstall the plug, crank the car and drive around for a few miles. Then drain again. You might want to do this a few times the first time. Be prepaid to buy at least 12 quarts. I would buy 16 just to make sure you can get as much of the old fluid out as possible.
Now Ford recommends that you drain and refill with 4 quarts every 30k I think it is. That is what I was told by the service manager at my local Ford dealer, but I trust the folks around here a lot more. Most here will agree that doing this every 15k is about right. Exspecially if you drive your car hard.
Since your car only has 50k on it, I will let you and others be the judge of that. If it were me, I would do what mentioned above, but that might just be me.
Also, there are some lube service places that offer transmission flushing services that you might want to look into. If you like, you can do a search in this forum using the words "flush" or "change" or something like that. I have never had this done, so I cannot help you out much here but there has been a good bit of discussion on this topic here in the past.
When ATF breaks down, I am not sure if it actually becomes thicker - such as motor oil does, but it does become abrasive. This can cause all sorts of problems as you can imagine.
As for your problem, is your check engine light lit up or does your O/D light (overdrive) flash? If so, you can go to somewhere like Autozone and get it scanned. Bring the code back here and give it to us.
