Why is transmission fluid the enemy of my '98 Lincoln Continental?

I had the fluid changed about a year ago and it shortly stopped working in gear, then in reverse. Eventually it was fixed, and driven 2000 miles to another state. The car worked perfectly for 5000 miles. My brother changed the transmission fluid and it's having the same problem again. It was shaking under 1500rpm yesterday, and today it only works in reverse. I tried googling it, and while I found some potential answers, I'm hoping someone will have a little more specific suggestion.
 
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